Disassociation of bone resorption and formation by GLP-2: a 14-day study in healthy postmenopausal women

Cytokines and Hormones That Contribute to the Positive Association between Fat and Bone

The positive association between body weight and bone density has been established in numerous laboratory and clinical studies. Apart from the direct effect of soft tissue mass on bone through skeletal loading, a number of cytokines and hormones contribute to the positive association between adipose and bone tissue, acting either locally in sites where cells of the two tissues are adjacent to each other or systemically through the circulation. The current review describes the effects of such local and systemic factors on bone physiology. One class of factors are the adipocyte-secreted peptides (adipokines), which affect bone turnover through a combination of direct effects in bone cells and indirect mechanisms mediated by the central nervous system. Another source of hormones that contribute to the coupling between fat and bone tissue are beta cells of the pancreas. Insulin, amylin, and preptin are co-secreted from pancreatic beta cells in response to increased glucose levels after feeding, and are also found in high circulating levels in obesity. A number of peptide hormones secreted from the gastrointestinal tract in response to feeding affect both fat and bone cells and thus can also act as mediators of the association between the two tissues. The current review focuses on results of laboratory studies investigating possible mechanism involved in the positive association between fat mass and bone mass.

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.

Effect of supplementary calcium phosphate on plasma gastrointestinal hormones in a double-blind, placebo-controlled, cross-over human study

Gastrointestinal hormones and Ca are associated with bone metabolism. The objective of the present human study was to determine the effect of calcium phosphate on the postprandial circulation of gastrointestinal hormones. A total of ten men participated in the present double-blind, placebo-controlled, cross-over study. The participants were divided into two groups. Of these, one group consumed bread enriched with 1 g Ca (pentacalcium hydroxy-triphosphate, CaP) daily for 3 weeks. The other group consumed placebo bread. After 2 weeks of washout, the intervention was changed between the groups for another 3 weeks. The subjects consumed a defined diet at the beginning (single administration) and at the end (repeated administration) of the intervention periods, and blood samples were drawn at 0, 30, 60, 120, 180 and 240 min. Between 0 and 30 min, the participants consumed a test meal, with or without CaP. The concentrations of gastrointestinal hormones (glucose-dependent insulinotropic polypeptide, glucagon-like peptide (GLP) 1 and GLP2), insulin and glucose were determined. The AUC of GLP1 (total and active) and GLP2 increased significantly after the repeated CaP administrations compared with that after placebo administration. The AUC of insulin and glucose showed no differences between the CaP and placebo administrations. CaP affects the postprandial plasma concentrations of gastrointestinal hormones through the modulation of the intestinal environment, e.g. bile acids and microbiota.

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.

Total and carboxylated osteocalcin associate with insulin levels in young adults born with normal or very low birth weight

OBJECTIVE

Osteocalcin (OC), a bone-derived protein, has been implicated in the regulation of glucose and energy metabolism. Young adults born with very low birth weight (VLBW) have altered glucose regulation and lower bone mineral density (BMD) compared with those born at term. The aim of this study was to explore the association between bone and glucose metabolism in healthy young adults born prematurely or at term.

METHODS

The cohort of this cross-sectional study comprised 332 non-diabetic young adults (age 18 to 27 years) born either preterm with VLBW (n = 163) or at term (n = 169). OC, carboxylated osteocalcin (cOC) and markers of glucose metabolism were measured at fasting and after a 75-g oral glucose tolerance test (OGTT).

RESULTS

VLBW adults were shorter, had lower BMD (p<0.001) and higher fasting OC (p = 0.027) and cOC (p = 0.005) than term-born subjects. They also had higher 2-hour insulin (p = 0.001) and glucose (p = 0.037) concentrations. OGTT induced a significant reduction in OC (p<0.001), similar in both groups. OC reduction was not associated with OGTT-induced increases in insulin (p = 0.54). However, fasting total OC and cOC correlated negatively with fasting insulin after adjustment for age, gender, BMD and VLBW status (r = -0.182, p = 0.009 and r = -0.283, p<0.001, respectively).

CONCLUSION

Adults born with VLBW have higher OC and cOC than their peers born at term. This may in part reflect the mechanisms that underlie their lower BMD and decreased insulin sensitivity. Serum OC appears to be negatively associated with long-term glucose regulation whereas acute changes during OGTT may be mediated via other mechanisms.

Recommendations on the management of fragility fracture risk in women younger than 70 years

The risk for fragility fracture represents a problem of enormous magnitude. It is estimated that only a small fraction of women with this risk take the benefit of preventive measures. The relationship between estrogen and bone mass is well known as they are the other factors related to the risk for fracture. There are precise diagnostic methods, including a tool to diagnose the risk for fracture. Yet there continues to be an under-diagnosis, with the unrecoverable delay in instituting preventive measures. Women under the age of 70 years, being much more numerous than those older, and having risk factors, are a group in which it is essential to avoid that first fragility fracture. Today it is usual not to differentiate between the treatment and the prevention of osteoporosis since the common aim is to prevent fragility fractures. Included in this are women with osteoporosis or with low bone mass and increased risk for fracture, for whom risk factors play a primary role. There is clearly controversy over the type of treatment and its duration, especially given the possible adverse effects of long-term use. This justifies the concept of sequential treatment, even more so in women under the age of 70, since they presumably will need treatment for many years. Bone metabolism is age-dependent. In postmenopausal women under 70 years of age, the increase in bone resorption is clearly predominant, related to a sharp drop in estrogens. Thus a logical treatment is the prevention of fragility fractures by hormone replacement therapy (HRT) and, in asymptomatic women, selective estradiol receptor modulators (SERMs). Afterwards, there is a period of greater resorption, albeit less intense but continuous, when one could utilise anti-resorptive treatments such as bisphosphonates or denosumab or a dual agent like strontium ranelate. Bone formation treatment, such as parathyroid hormone (PTH), in women under 70 years will be uncommon. That is because it should be used in cases where the formation is greatly diminished and there is a high risk for fracture, something found in much older women.

Measurement of C-terminal telopeptide of type I collagen (CTX) in serum

Serum CTX assays measure a fragment of the C-terminal telopeptide of type 1 collagen released during resorption of mature bone. Assay reagents are available in manual and automated formats and give good analytical performance. However their standardisation is not transparent and significant differences in results between methods have been demonstrated. CTX is most stable in EDTA plasma, although serum samples processed promptly would be satisfactory. sCTX shows a profound circadian rhythm, especially in non-fasting subjects; specimens should be collected from fasting patients at a well-defined time of day to minimise biological variation. Reference intervals in pre-menopausal women have been well studied but in other adult groups there is less information. Healthy children show the expected age-related variation corresponding to growth rate. Serum CTX fulfils or partially fulfils all the criteria of a reference bone turnover marker. Further studies aimed at reducing inter-method differences in results and establishing the relationships of sCTX with fracture risk and with fracture risk improvement with treatment are required.

Emerging therapies for osteoporosis

This article reviews the conceptual framework for agents that are antiresorptive or anabolic, including pathways that affect bone formation and resorption, and the steps in those pathways that are targets for new therapeutic agents. This article discusses novel antiresorptive and anabolic agents in development. Recent developments that link bone remodeling with serotonin in the gastrointestinal system and the central nervous system via the sympathetic nervous system may change the paradigm for skeletal remodeling. Novel anabolic agents in development include antibodies that target molecules involved in Wnt signaling.

Safety of Anti-Diabetic Therapies on Bone

Osteoporosis and diabetic disease have reached epidemic proportion and create significant public health concerns. The prevalence of these diseases is alarming, and indicates that in the US, 50% of elderly individuals are osteoporotic and almost 20% of population has either diabetic or prediabetic conditions (Centers for Disease Control and Prevention; http://www.cdc.gov). Osteoporosis and diabetes share many features including genetic predispositions and molecular mechanisms. The linkage between these two chronic diseases, which stems from overlapping molecular controls involved in bone homeostasis and energy metabolism, creates a possibility that certain anti-diabetic therapies may affect bone. This concurs with recent findings indicating that bone status is closely linked to regulation of energy metabolism and insulin sensitivity. Indeed, bone and energy homeostasis are under the control of the same regulatory factors, including insulin, peroxisome proliferator activated receptor gamma (PPARγ), gastrointestinal hormones such as glucose inhibitory protein (GIP) and glucagon inhibitory peptide (GLP), and bone derived hormone osteocalcin. These factors and related mechanisms control glucose homeostasis and fatty acids metabolism in fat tissue, pancreas and intestine, which are pharmacological targets for anti-diabetic therapies. The same factors contribute to the bone quality by their effect on bone cell differentiation and bone remodeling process. This implies that bone should be considered as a vital target for therapies which modulate energy metabolism. This review is summarizing available data on the skeletal effects of clinically approved anti-diabetic therapies.

The effect of oral glucose tolerance test on serum osteocalcin and bone turnover markers in young adults

Osteocalcin (OC) is an osteoblast-derived protein implicated in the regulation of glucose tolerance and energy metabolism. This endocrine function has been suggested to be exerted via its undercarboxylated form, which has been shown to induce expression of adiponectin, insulin, and islet cell proliferation in mice. Furthermore, insulin has recently been shown to regulate the biological activity of OC in bone. Our aim was to explore the association between glucose and bone metabolism by evaluating the effect of a standard 75 g oral glucose tolerance test (OGTT) on serum OC, carboxylated OC (cOC) and bone-turnover markers (BTMs) C terminal telopeptide (βCTX-I) and N terminal propeptide (PINP) of type I collagen and tartrate-resistant acid phosphatase 5b (TRACP5b). Serum samples collected at 0 and at 120 min were analyzed in a cohort of normoglycemic young adults (n = 23, mean age 23.6 years). During OGTT a significant decrease was observed in all BTMs (P < 0.001 for all variables). The median decreases from 0 to 120 min for OC, cOC, βCTX-I, PINP, and TRACP5b were -32.1% (-37.9 to -19.6), -34.4% (-39.8 to -22.2), -61.4% (-68.5 to -53.0), -26.8% (-33.2 to -19.2), and -44.5% (-48.3 to -40.2), respectively. A strong association between the changes in OC and cOC was observed (r = 0.83, P < 0.001). The decrease in PINP was associated with changes in OC, whereas the changes in βCTX-I and TRACP5b were not associated with decreases in OC or cOC. The observed OGTT-induced changes in bone-derived proteins were partially independent of each other and potentially mediated by different mechanisms.

Use of Bone Turnover Markers in Clinical Osteoporosis Assessment in Women: Current Issues and Future Options

Monitoring bone turnover of the adult and aging skeleton is essential for optimal treatment of bone metabolic diseases, such as postmenopausal osteoporosis. Diagnosis of osteoporosis is based solely on dual-emission x-ray absorptiometry-based measurements of bone mineral density. However, within the last 20 years, biochemical markers of bone turnover have been implemented to a larger degree, and especially within the field of drug development. Numerous clinical studies have underscored that the markers have promise in terms of predicting patients at high risk of losing bone, future fracture events and importantly also the fracture efficacy of drugs in development. Furthermore, while classical methods often require years to monitor the changes, the bone turnover markers do so within a shorter time span. The aims of this article are to provide an update on the different biochemical markers of bone turnover, and to give an overview of their applications in epidemiological and clinical research especially in women. The main emphasis will be on their utility in clinical trials testing the efficacy of drugs for the treatment of osteoporosis, and their ability to supplement bone mass measurements. Finally, recent evidence suggests that biochemical markers may provide information on bone age that may indirectly relate to bone quality, and this is discussed together with future possibilities for measuring bone quality using bone turnover markers. In summary, a more targeted use of biomarkers could assist in the identification of high-risk patients, the process of drug discovery and monitoring of the efficacy of osteoporosis treatment in clinical settings.

Receptors and effects of gut hormones in three osteoblastic cell lines

Background

In recent years the interest on the relationship of gut hormones to bone processes has increased and represents one of the most interesting aspects in skeletal research. The proportion of bone mass to soft tissue is a relationship that seems to be controlled by delicate and subtle regulations that imply "cross-talks" between the nutrient intake and tissues like fat. Thus, recognition of the mechanisms that integrate a gastrointestinal-fat-bone axis and its application to several aspects of human health is vital for improving treatments related to bone diseases. This work analysed the effects of gut hormones in cell cultures of three osteoblastic cell lines which represent different stages in osteoblastic development. Also, this is the first time that there is a report on the direct effects of glucagon-like peptide 2, and obestatin on osteoblast-like cells.

Methods

mRNA expression levels of five gut hormone receptors (glucose-dependent insulinotropic peptide [GIP], glucagon-like peptide 1 [GLP-1], glucagon-like peptide 2 [GLP-2], ghrelin [GHR] and obestatin [OB]) were analysed in three osteoblastic cell lines (Saos-2, TE-85 and MG-63) showing different stages of osteoblast development using reverse transcription and real time polymerase chain reaction. The responses to the gut peptides were studied using assays for cell viability, and biochemical bone markers: alkaline phosphatase (ALP), procollagen type 1 amino-terminal propeptides (P1NP), and osteocalcin production.

Results

The gut hormone receptor mRNA displayed the highest levels for GIP in Saos-2 and the lowest levels in MG-63, whereas GHR and GPR39 (the putative obestatin receptor) expression was higher in TE-85 and MG-63 and lower in Saos-2. GLP-1 and GLP-2 were expressed only in MG-63 and TE-85. Treatment of gut hormones to cell lines showed differential responses: higher levels in cell viability in Saos-2 after GIP, in TE-85 and MG-63 after GLP-1, GLP-2, ghrelin and obestatin. ALP showed higher levels in Saos-2 after GIP, GHR and OB and in TE-85 after GHR. P1NP showed higher levels after GIP and OB in Saos-2. Decreased levels of P1NP were observed in TE-85 and MG-63 after GLP-1, GLP-2 and OB. MG-63 showed opposite responses in osteocalcin levels after GLP-2.

Conclusions

These results suggest that osteoblast activity modulation varies according to different development stage under different nutrition related-peptides.

Energy Balance, Myostatin, and GILZ: Factors Regulating Adipocyte Differentiation in Belly and Bone

Peroxisome proliferator-activated receptor gamma (PPAR-gamma) belongs to the nuclear hormone receptor subfamily of transcription factors. PPARs are expressed in key target tissues such as liver, fat, and muscle and thus they play a major role in the regulation of energy balance. Because of PPAR-gamma's role in energy balance, signals originating from the gut (e.g., GIP), fat (e.g., leptin), muscle (e.g., myostatin), or bone (e.g., GILZ) can in turn modulate PPAR expression and/or function. Of the two PPAR-gamma isoforms, PPAR-gamma2 is the key regulator of adipogenesis and also plays a role in bone development. Activation of this receptor favors adipocyte differentiation of mesenchymal stem cells, while inhibition of PPAR-gamma2 expression shifts the commitment towards the osteoblastogenic pathway. Clinically, activation of this receptor by antidiabetic agents of the thiazolidinedione class results in lower bone mass and increased fracture rates. We propose that inhibition of PPAR-gamma2 expression in mesenchymal stem cells by use of some of the hormones/factors mentioned above may be a useful therapeutic strategy to favor bone formation.

Obesity, diabetes mellitus and last but not least, osteoporosis

Knowledge about the influence of bone on intermediary metabolism corresponds to a developing area that has gained prominence. The old concept of bone and adipose tissues as inert metabolic tissues, with minor contributions to metabolic adaptations has been reconsidered in light of findings that bone is involved in the development of insulin sensitivity. Similarly adipose tissue exerts important influences on bone mass development and maintenance. Moreover, the use of drugs in the treatment of metabolic disorders such as diabetes mellitus can impact bone metabolism. These networks linking osteoporosis to obesity and diabetes mellitus have reinvigorated investigations in the pathophysiology of osteoporosis. The present review examines this aspect and calls attention to health care providers and potential treatments of skeletal disorder.

Osteoclast activity and subtypes as a function of physiology and pathology--implications for future treatments of osteoporosis

Osteoclasts have traditionally been associated exclusively with catabolic functions that are a prerequisite for bone resorption. However, emerging data suggest that osteoclasts also carry out functions that are important for optimal bone formation and bone quality. Moreover, recent findings indicate that osteoclasts have different subtypes depending on their location, genotype, and possibly in response to drug intervention. The aim of the current review is to describe the subtypes of osteoclasts in four different settings: 1) physiological, in relation to turnover of different bone types; 2) pathological, as exemplified by monogenomic disorders; 3) pathological, as identified by different disorders; and 4) in drug-induced situations. The profiles of these subtypes strongly suggest that these osteoclasts belong to a heterogeneous cell population, namely, a diverse macrophage-associated cell type with bone catabolic and anabolic functions that are dependent on both local and systemic parameters. Further insight into these osteoclast subtypes may be important for understanding cell-cell communication in the bone microenvironment, treatment effects, and ultimately bone quality.

New treatment targets in osteoporosis

Postmenopausal osteoporosis is characterized by bone remodeling alterations with an imbalance between excessive bone resorption and inadequate bone formation. At present, osteoporosis treatment rests on bone resorption inhibitors and, more specifically, on bisphosphonates. However, the introduction of anabolic agents such as parathyroid hormone that stimulate bone formation has expanded the range of treatment options. New treatment targets have been identified via improved knowledge on bone pathophysiology, bone remodeling, bone cells and intracellular signaling pathways. RANKL inhibition by anti-RANKL antibodies is undergoing considerable development as a treatment for osteoporosis. Also under development are anti-catabolic drugs that target the molecular mechanisms involved in bone resorption, including cathepsin K inhibitors and integrin alpha(v)beta(3) antagonists. The identification of new pathways involved in bone formation is directing clinical research efforts toward the development of anabolic agents. The signaling pathways involved in bone formation, most notably the Wnt-pathway, hold considerable promise as treatment targets in conditions characterized by insufficient bone formation. Current focuses of interest include antibodies against naturally occurring Wnt-pathway antagonists (e.g., sclerostin and Dkk1) and modulators of parathyroid hormone production (calcilytic agents). Thus, active research is ongoing to improve the treatment of osteoporosis, a disease whose high prevalence and considerable functional and socioeconomic impact will raise formidable challenges in the near future.

The effect of glucagon-like peptide-2 on arterial blood flow and cardiac parameters

BACKGROUND

Glucagon-like peptide-2 (GLP-2) is known to increase mesenteric blood flow. The aim of the study was to evaluate the effect of GLP-2 on blood flow in different vascular sites, and dynamic changes in cardiac parameters.

METHODS

10 healthy volunteers were given 450 nmol subcutaneous (SC) GLP-2 or isotonic saline (5 subjects) in a single blinded manner. During the following 90 min, blood flow in the superior mesenteric artery (SMA), celiac artery (CA), renal artery (RA), common carotid artery (CCA) was measured using Doppler ultrasound (US), and cardiovascular variables were measured by impedance cardiography and finger plethysmography. Plasma GLP-2 was measured at times 0, 30 and 60 min.

RESULTS

Compared to the placebo group, GLP-2 elicited a 27% decrease in the resistance index (RI) and a 269.4% increase in Time Averaged Maximal Velocity (TAMV) in the SMA (P<0.01). CA, RA and CCA: There were no significant changes in RI or TAMV in the GLP-2 or placebo group, and no change in CA diameter. Cardiac parameters: GLP-2 increased cardiac output (CO), stroke volume (SV) and heart rate (HR) compared to baseline (respectively: 15.3, 4.81 and 8.2% (P<0.001, P<0.01 and P<0.01)). The CO, SV and HR changes were not significantly different from the placebo group. Mean plasma GLP-2 serum levels in the placebo group at times 0, 30 and 60 min were 22.8, 23.4 and 23.2 pmol/l. In the GLP-2 group 20.3, 1273 and 1725 pmol/l.

CONCLUSION

SC GLP-2 increased SMA blood flow, as previously shown, but elicited no changes in other vascular sites. CO and HR increased significantly, presumably due to the increased mesenteric blood flow.

Gastrointestinal peptides and bone health

PURPOSE OF REVIEW

To outline recent developments in research surrounding gastrointestinal peptides and their role in skeletal regulation.

RECENT FINDINGS

Bone remodeling is influenced by many regulatory systems, which interact to ensure that the complex demands upon mineralized tissue are met without undue compromise. These include local actions such as mechanical factors, but are dominated by systemic endocrine factors. Although the involvement of hypothalamo-pituitary actions on bone homeostasis is well defined, growing evidence suggests that peripheral tissues and the circulating factors they produce represent an important regulatory axis in bone. Given the critical role of diet in mineral homeostasis, the gastrointestinal tract is a rich source of circulating factors capable of regulating bone homeostasis. After a review of manuscripts on known mechanisms and effects of gastrointestinal peptide on bone, these were summarized.

SUMMARY

Although clearly an exciting and emergent field of research, more studies are required to define the specific actions of gastrointestinal regulator in bone, in particular, the relative contribution of systemic and local effects, to aid interpretation of their potential impact on human health and disease. Nonetheless, this exciting research will further our understanding on bone physiology and provide novel approaches to therapy in a wide range of skeletal conditions.

Molecular biology of bone remodeling: implications for new therapeutic targets for osteoporosis

Osteoporosis is a major public health problem for adults over age 55 years costing billions of euros/dollars. Over the last 20 years anti-resorptive drugs were the treatment of choice for osteoporosis and most were derived from the bisphosphonate molecule. In the last 7 years remarkable advances in molecular biology and genetics have led to a detailed understanding of the bone remodeling cycle and as a result new therapeutic targets for treatment emerged. These new compounds have different modes of action depending on their role in the bone remodeling cycle. A major discovery was the important role of RANKL (receptor activator of nuclear factor kappa B ligand) secreted by osteoblasts and responsible for stimulating osteoclastic bone resorption. This led to development of a potent monoclonal antibody that blocks its action. This drug should be available soon as a new treatment for osteoporosis. Other molecular targets in resorption have been identified and several specific antagonists are potential treatments. However, a significant limiting factor for a new anti-resorptive drug is the cost of bringing it to the market because of the huge costs of a fracture trial. Although anti-resorptive agents have been the backbone of osteoporosis treatment they do not rebuild bone architecture and development of anabolic agents is needed. These are likely to evolve from an understanding of the LRP/Wnt signaling pathway. Already an antibody against sclerostin has shown promise in animal studies, and not to forget parathyroid hormone which was the first clinically useful anabolic treatment for osteoporosis.

Four-month treatment with GLP-2 significantly increases hip BMD: a randomized, placebo-controlled, dose-ranging study in postmenopausal women with low BMD

We have previously shown that repeated dosing of glucagon-like peptide-2 (GLP-2) at 10 p.m. in postmenopausal women for 14 days results in a dose-dependent decrease in the nocturnal bone resorption, as assessed by s-CTX. In contrast, bone formation, as assessed by serum osteocalcin, appeared to be unaffected by treatment with exogenous GLP-2, at least over 14 days. The present study extends the observation period to four months. The study was a double-blind placebo-controlled dose-ranging trial comparing three different doses of GLP-2 (0.4 mg, 1.6 mg and 3.2 mg GLP-2, administered nightly) against a saline control injection. We examined safety and tolerability, and the effects on biochemical markers of bone turnover and the effect on bone mineral density. Injection of 0.4 mg, 1.6 mg and 3.2 mg GLP-2 resulted in similar reduction in the nocturnal rise of s-CTX, at Treatment Day 120 the mean difference to placebo was approximately -150%*h at AUC(0-10H) (P<0.01). Osteocalcin levels were unaffected in the 10-hour period after injection indicating that injections of 0.4 mg, 1.6 mg and 3.2 mg GLP-2 do not exert any acute stimulatory or inhibitory effect on bone formation. Treatment with GLP-2 resulted in a significant dose-dependent increase in total hip BMD over the course of the study that for the 3.2 mg GLP-2 group reached 1.1% (P=0.007) from baseline. The overall rates of adverse events in the 4 treatment groups were similar and there were no signs of tachyphylaxis or antibodies against GLP-2. The results indicate that GLP-2 produces a substantial decrease in bone resorption without suppression of bone formation thereby changing the bone remodeling balance in favor of bone formation, particularly at the hip.

Future therapeutic targets in osteoporosis

PURPOSE OF REVIEW

To describe new agents for the treatment of osteoporosis, discuss a conceptual framework of agents that are antiresorptive or anabolic, and review pathways that affect bone turnover and steps in those pathways that are targets for new therapeutic agents.

RECENT FINDINGS

Novel antiresorptive agents are being developed. Denosumab, a fully human mononoclonal antibody to receptor activator of nuclear factor kappa B ligand, has completed its major fracture trial. Assessment of odanacatib, an inhibitor of cathepsin K, an osteoclast enzyme required for resorption of bone matrix, is underway. Glucagon-like peptide 2 is an intestinal peptide that prevents the nocturnal rise in bone resorption. Anabolic agents act by stimulating new bone formation. Novel anabolic agents in development include antibodies that target molecules (sclerostin and Dkk1) involved in Wnt signaling, a pathway that regulates gene transcription of proteins that are important for osteoblast function. An antagonist to the calcium-sensing receptor and an activin receptor fusion protein, which functions as an activin antagonist, have shown promise as anabolic agents in early human trials.

SUMMARY

This review discusses potential future advances in drug therapy for osteoporosis including novel antiresorptive and anabolic agents that may become available in the coming years.

Short bowel patients treated for two years with glucagon-like Peptide 2: effects on intestinal morphology and absorption, renal function, bone and body composition, and muscle function

BACKGROUND AND AIMS

In a short-term study, Glucagon-like peptide 2 (GLP-2) has been shown to improve intestinal absorption in short bowel syndrome (SBS) patients. This study describes longitudinal changes in relation to GLP-2 treatment for two years.

METHODS

GLP-2, 400 micrograms, s.c.,TID, were offered, to eleven SBS patients keeping parenteral support constant. 72-hour nutritional balance studies were performed at baseline, weeks 13, 26, 52 during two years intermitted by an 8-week washout period. In addition, mucosal morphometrics, renal function (by creatinine clearance), body composition and bone mineral density (by DEXA), biochemical markers of bone turnover (by s-CTX and osteocalcin, PTH and vitamin D), and muscle function (NMR, lungfunction, exercise test) were measured.

RESULTS

GLP-2 compliance was >93%. Three of eleven patients did not complete the study. In the remaining 8 patients, GLP-2 significantly reduced the fecal wet weight from approximately 3.0 to approximately 2.0 kg/day. This was accompanied by a decline in the oral wet weight intake, maintaining intestinal wet weight absorption and urinary weight constant. Renal function improved. No significant changes were demonstrated in energy intake or absorption, and GLP-2 did not significantly affect mucosal morphology, body composition, bone mineral density or muscle function.

CONCLUSIONS

GLP-2 treatment reduces fecal weight by approximately 1000 g/d and enables SBS patients to maintain their intestinal fluid and electrolyte absorption at lower oral intakes. This was accompanied by a 28% improvement in creatinine clearance.

Effect of GLP-1 treatment on bone turnover in normal, type 2 diabetic, and insulin-resistant states

It has been suggested that hormones released after nutrient absorption, such as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 2 (GLP-2), could be responsible for changes in bone resorption. However, information about the role of GLP-1 in this regard is scanty. Diabetes-related bone loss occurs as a consequence of poor control of glucose homeostasis, but the relationship between osteoporosis and type 2 diabetes remains unclear. Since GLP-1 is decreased in the latter condition, we evaluated some bone characteristics in streptozotocin-induced type 2 diabetic (T2D) and fructose-induced insulin-resistant (IR) rat models compared to normal (N) and the effect of GLP-1 or saline (control) treatment (3 days by osmotic pump). Blood was taken before and after treatment for plasma measurements; tibiae and femora were collected for gene expression of bone markers (RT-PCR) and structure (microCT) analysis. Compared to N, plasma glucose and insulin were, respectively, higher and lower in T2D; osteocalcin (OC) and tartrate-resistant alkaline phosphatase 5b were lower; phosphate in IR showed a tendency to be higher; PTH was not different in T2D and IR; all parameters were unchanged after GLP-1 infusion. Bone OC, osteoprotegerin (OPG) and RANKL mRNA were lower in T2D and IR; GLP-1 increased OC and OPG in all groups and RANKL in T2D. Compared to N, trabecular bone parameters showed an increased degree of anisotropy in T2D and IR, which was reduced after GLP-1. These findings show an insulin-independent anabolic effect of GLP-1 and suggest that GLP-1 could be a useful therapeutic agent for improving the deficient bone formation and bone structure associated with glucose intolerance.

Glucagon-like peptide-2 increases mesenteric blood flow in humans

OBJECTIVE

Mesenteric blood flow is believed to be influenced by digestion and absorption of ingested macronutrients. We hypothesized that the intestinotrophic hormone, GLP-2 (glucagons-like peptide 2), may be involved in the regulation of mesenteric blood flow. Changes in mesenteric blood flow were measured by Doppler ultrasound scanning of the superior mesenteric artery (SMA). The aim of the study was to demonstrate the influence of GLP-2 on this flow, expressed as changes in resistance index (RI).

MATERIAL AND METHODS

A homogeneous group of 10 fasting healthy volunteers completed a 2-day trial. On day 1, a standard meal was given, and RI measured in the SMA. On day 2, GLP-2 was infused intravenously (IV) at rates of 0.5, 1.0 and 2.0 pmol/kg/min over 3 x 45 min separated by a 15-20 min rest period. After a further 15-20 min of rest, 450 nmol synthetic GLP-2 was given subcutaneously (SC). RI in the SMA was measured before, during and after the meal and GLP-2 infusions.

RESULTS

After IV infusion of GLP-2, the following decreases in RI were observed: 0.5 pmol/kg/min: 2.7% (range 0-6.3%), 1.0 pmol/kg/min: 6.7% (range 0.4-15.9%), 2.0 pmol/kg/min: 15.3% (range 9.6-22.7%) p<0.00802. When given SC, GLP-2 elicited a maximum average change in RI of 15.6% (range 5.0-28.1%). The standard meal elicited a 14.7% (range 8.8-21.6%) change, p<0.020 There was a similar change in RI over time (0-90 min) after a standard meal and after subcutaneous GLP-2, p<0.005.

CONCLUSIONS

Our study showed a significant association between IV and SC administration of synthetic GLP-2 and changes in mesenteric blood flow. An exponential dose-response relationship was observed after IV infusion. The meal-induced changes in mesenteric blood flow over time were similar to those obtained by SC GLP-2. Thus, our results support the hypothesis that GLP-2 is an important regulator of mesenteric blood flow.

Effects of treatment with glucagon-like peptide-2 on bone resorption in colectomized patients with distal ileostomy or jejunostomy and short-bowel syndrome

OBJECTIVE

The gut hormone GLP-2 (glucagon-like peptide-2) seems to be involved in the circadian pattern of bone resorption, whereas parathyroid hormone (PTH) is an established key hormone in bone turnover. Endogenous GLP-2 secretion is lacking in colectomized patients with short-bowel syndrome (SBS) and they have reduced bone mineral density (BMD). The aim of the study was to investigate the anti-resorptive effect (assessed by s-CTX) of 14 days of GLP-2 treatment in these patients and to determine whether 56 days of treatment would improve BMD. PTH secretion in response to GLP-2 was also investigated in colectomized SBS patients and colectomized controls (with ileostomy).

MATERIAL AND METHODS

Eight SBS patients and 13 patients with ileostomy were treated with subcutaneous injections of 1600 microg GLP-2 at bedtime for 56 and 14 consecutive days, respectively. BMD was determined at days 1 and 56 in SBS patients. On days 1 and 14, measurements of CTX, P1NP and PTH were taken 4 h after the GLP-2 injection.

RESULTS

Patients with ileostomy showed a significant reduction in bone resorption after GLP-2 injections at days 1 and 14. In contrast, there was no change in s-CTX after 1 and 14 days in the SBS patients, and after 56 days of GLP-2 treatment there was no improvement in BMD. A significant reduction in PTH secretion in response to GLP-2 was observed only in patients with ileostomy.

CONCLUSIONS

The decreased bone resorption in response to GLP-2 injections cannot be elicited in SBS patients and therefore precludes treatment of their osteopenia with GLP-2. The anti-resorptive response to GLP-2 seems to require an intact small intestine and may involve suppression of PTH secretion.

Reduction in bone resorption by exogenous glucagon-like peptide-2 administration requires an intact gastrointestinal tract

OBJECTIVE

Biochemical markers for bone resorption (s-CTX) are reduced by food intake, whereas markers for bone formation seem to be unaffected by meal status. Glucagon-like peptide-2 (GLP-2) is a peptide secreted from endocrine L cells in the intestinal mucosa in relation to food-intake. Subcutaneous GLP-2 treatment has been shown to reduce bone resorption in postmenopausal women. The objective of this study was to investigate the ability of exogenous GLP-2 to reduce bone resorption in patients with jejunostomy or ileostomy and to elucidate whether an intact gastrointestinal tract and the ability to secrete GLP-2 are required for meal-induced inhibition of bone resorption.

MATERIAL AND METHODS

Fifteen control subjects, 13 colectomized patients with an ileostomy and 12 colectomized patients with a jejunostomy (remnant small bowel 89 +/- 53 cm) were given: a) a subcutaneous injection of 1600 microg GLP-2, b) placebo and c) 3.8 MJ of a breakfast meal. Blood was sampled for measurements of s-CTX, s-osteocalcin and GLP-2 for 4 h after each intervention.

RESULTS

After the GLP-2 injection, only control subjects showed a significant reduction in s-CTX (24% +/- 13%, p = 0.05, 120 min) compared with baseline values. Patients with an ileostomy had a preserved endogenous postprandial GLP-2 secretion, which was absent in patients with a jejunostomy. Consumption of a meal reduced s-CTX in all groups but significantly less so in the jejunostomy group.

CONCLUSIONS

Reductions in bone resorption by exogenous GLP-2 require an intact gastrointestinal tract. The decreased meal-induced inhibition of bone resorption in the jejunostomy patients, who lack a GLP-2 response, supports the view that GLP-2 plays a role in postprandial reduction in bone resorption.

The murine glucagon-like peptide-1 receptor is essential for control of bone resorption

Gastrointestinal hormones including gastric inhibitory polypeptide (GIP), glucagon-like peptide (GLP)-1, and GLP-2 are secreted immediately after meal ingestion, and GIP and GLP-2 have been shown to regulate bone turnover. We hypothesize that endogenous GLP-1 may also be important for control of skeletal homeostasis. We investigated the role of GLP-1 in the regulation of bone metabolism using GLP-1 receptor knockout (Glp-1r(-/-)) mice. A combination of bone density and histomorphometry, osteoclast activation studies, biochemical analysis of calcium and PTH, and RNA analysis was used to characterize bone and mineral homeostasis in Glp-1r(-/-) and Glp-1r(+/+) littermate controls. Glp-1r(-/-) mice have cortical osteopenia and bone fragility by bone densitometry as well as increased osteoclastic numbers and bone resorption activity by bone histomorphometry. Although GLP-1 had no direct effect on osteoclasts and osteoblasts, Glp-1r(-/-) mice exhibited higher levels of urinary deoxypyridinoline, a marker of bone resorption, and reduced levels of calcitonin mRNA transcripts in the thyroid. Moreover, calcitonin treatment effectively suppressed urinary levels of deoxypyridinoline in Glp-1r(-/-), mice and the GLP-1 receptor agonist exendin-4 increased calcitonin gene expression in the thyroid of wild-type mice. These findings establish an essential role for endogenous GLP-1 receptor signaling in the control of bone resorption, likely through a calcitonin-dependent pathway.

Variations along the 24-hour cycle of circulating osteoprotegerin and soluble RANKL: a rhythmometric analysis

The variability of serum osteoprotegerin (OPG) and soluble RANKL (sRANKL) along the 24-h cycle was assessed in 20 healthy women. No rhythmic variations of serum OPG, sRANKL or sRANKL/OPG ratio were detected as a group phenomenon. Timing of sampling is unlikely to influence the results of measurements of circulating OPG and sRANKL.

INTRODUCTION

Physiological bone turnover shows diurnal variations. The aim of the study was to assess variability of OPG and sRANKL serum levels along the 24-h cycle.

METHODS

Blood was collected from 20 healthy women (median age 31 years, range 25-65 years) at 4-h intervals between 08:00 and 24:00 and at 2-h intervals between 24:00 and 08:00. Serum albumin, cortisol, osteocalcin (OC), C-terminal telopeptide of type I collagen (CTX), OPG and total sRANKL were measured. Temporal variations were assessed by the COSINOR model.

RESULTS

Circadian rhythms of cortisol and albumin documented a normal synchronization within the circadian structure. Serum OC and CTX showed rhythmic variations, peaking at night-time. Rhythmic variations of serum OPG, sRANKL and sRANKL/OPG ratio were not detected as a group phenomenon. On an individual basis, rhythmic changes were detected in ten patients for OPG and eight patients for sRANKL, with very small amplitudes and heterogeneous acrophases.

CONCLUSIONS

The absence of consistent rhythmic variations of circulating OPG and sRANKL levels may reflect the absence of rhythmic variations of their expression in the bone microenvironment. Were this the case, the nocturnal rise of bone resorption should be accounted for by different, not RANKL/OPG-mediated factors. Since circulating OPG and sRANKL may derive from sources other than bone, rhythmicity could be masked by non-rhythmic or non-synchronized rhythmic expression in these sources. Timing of sampling is unlikely to influence the results of measurements of circulating OPG and sRANKL.

The alpha cell expresses glucagon-like peptide-2 receptors and glucagon-like peptide-2 stimulates glucagon secretion from the rat pancreas

AIMS/HYPOTHESIS

Glucagon-like peptide-2 (GLP-2) is a gut hormone regulating intestinal growth and nutrient absorption. Recently, GLP-2 has been reported to stimulate glucagon secretion in healthy humans. We sought to clarify the mechanism and physiological significance of this endocrine effect.

MATERIALS AND METHODS

The expression of the GLP-2 receptor gene, Glpr2, and the localisation of the protein were evaluated by real-time PCR on cDNA from isolated rat islets and by immunohistochemistry in rat and human pancreas. The glucagon, insulin and somatostatin responses to 0.1, 1 and 10 nmol/l GLP-2 and to GLP-1 and GLP-2 given simultaneously were studied in the isolated perfused rat pancreas.

RESULTS

Expression of Glp2r transcript was confirmed by PCR. In both human and rat pancreas, GLP-2r immunoreactivity was colocalised with proglucagon. GLP-2 at 10 nmol/l increased glucagon secretion significantly from a pre-infusion level of 0.314 +/- 0.07 to 0.508 +/- 0.09 pmol/min (p < 0.0005), whereas lower GLP-2 concentrations were ineffective. Neither insulin nor somatostatin output was influenced. During simultaneous administration of GLP-1 and GLP-2, net glucagon release was no longer reduced by 0.1, 1 or 10 nmol/l GLP-1, which, when given alone, inhibited glucagon secretion by 25.0 +/- 9.9, 46.2 +/- 4.8, and 44.1 +/- 2.9%, respectively.

CONCLUSIONS/INTERPRETATION

Our results suggest that GLP-2 stimulates glucagon secretion through GLP-2r present on the alpha cell in rats. In the presence of GLP-2, the glucagonostatic effect of GLP-1, normally co-secreted with GLP-2, is markedly inhibited. Based on our analogous immunohistochemical findings in human pancreas, this mechanism also applies in all likelihood to humans. However, further in vivo studies are required to assess the physiological significance of the glucagonotropic action of GLP-2 in humans.