Potential role of pancreatic and enteric hormones in regulating bone turnover

Enhanced bone regeneration via endochondral ossification using Exendin-4-modified mesenchymal stem cells

Nonunions and delayed unions pose significant challenges in orthopedic treatment, with current therapies often proving inadequate. Bone tissue engineering (BTE), particularly through endochondral ossification (ECO), emerges as a promising strategy for addressing critical bone defects. This study introduces mesenchymal stem cells overexpressing Exendin-4 (MSC-E4), designed to modulate bone remodeling via their autocrine and paracrine functions. We established a type I collagen (Col-I) sponge-based in vitro model that effectively recapitulates the ECO pathway. MSC-E4 demonstrated superior chondrogenic and hypertrophic differentiation and enhanced the ECO cell fate in single-cell sequencing analysis. Furthermore, MSC-E4 encapsulated in microscaffold, effectively facilitated bone regeneration in a rat calvarial defect model, underscoring its potential as a therapeutic agent for bone regeneration. Our findings advocate for MSC-E4 within a BTE framework as a novel and potent approach for treating significant bone defects, leveraging the intrinsic ECO process.

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.

Circulatory Adipokines and Incretins in Adolescent Idiopathic Scoliosis: A Pilot Study

Adolescent idiopathic scoliosis (AIS) is a three-dimensional malformation of the spine of unknown cause that develops between 10 and 18 years old and affects 2-3% of adolescents, mostly girls. It has been reported that girls with AIS have a taller stature, lower body mass index (BMI), and bone mineral density (BMD) than their peers, but the causes remain unexplained. Energy metabolism discrepancies, including alterations in adipokine and incretin circulatory levels, could influence these parameters and contribute to disease pathophysiology. This pilot study aims to compare the anthropometry, BMD, and metabolic profile of 19 AIS girls to 19 age-matched healthy controls. Collected data include participants' fasting metabolic profile, anthropometry (measurements and DXA scan), nutritional intake, and physical activity level. AIS girls (14.8 ± 1.7 years, Cobb angle 27 ± 10°), compared to controls (14.8 ± 2.1 years), were leaner (BMI-for-age z-score ± SD: -0.59 ± 0.81 vs. 0.09 ± 1.11, p = 0.016; fat percentage: 24.4 ± 5.9 vs. 29.2 ± 7.2%, p = 0.036), had lower BMD (total body without head z-score ± SD: -0.6 ± 0.83 vs. 0.23 ± 0.98, p = 0.038; femoral neck z-score: -0.54 ± 1.20 vs. 0.59 ± 1.59, p = 0.043), but their height was similar. AIS girls had higher adiponectin levels [56 (9-287) vs. 32 (7-74) μg/mL, p = 0.005] and lower leptin/adiponectin ratio [0.042 (0.005-0.320) vs. 0.258 (0.024-1.053), p = 0.005]. AIS participants with a Cobb angle superior to 25° had higher resistin levels compared to controls [98.2 (12.8-287.2) vs. 32.1 (6.6-73.8), p = 0.0013]. This pilot study suggests that adipokines are implicated in AIS development and/or progression, but more work is needed to confirm their role in the disease.

Acute CTX-1 Suppression With Milk Calcium or Calcium Carbonate is Independent of Visceral Fat in A Randomized Crossover Study in Lean and Overweight Postmenopausal Women

BACKGROUND

Postmenopausal women with higher visceral adipose tissue (VAT) present with suppressed bone resorption (lower c-terminal cross-linking telopeptide of type I collagen, CTX-1) and turnover (lower osteocalcin) but whether this blunts the effect of calcium is unknown.

OBJECTIVE

The primary outcome of this study was the effect of VAT on changes in CTX-1 after intake of two forms of calcium. Secondary outcomes included changes in parathyroid hormone (PTH), serum calcium, phosphorus, and alkaline phosphatase (ALP).

DESIGN

Randomized open three period crossover trial conducted between 2017-2019 at the University of South Australia among 77 lean and overweight postmenopausal women (53-79 y) with body mass index (BMI) less than 25 kg/m2 and greater than 27 kg/m2, respectively. Participants received a single dose of milk (1000 mg calcium), calcium carbonate tablet (1000 mg calcium), and fruit juice (no calcium) in random order with a seven-day washout period. Blood samples were collected at baseline and hourly for 5h. Data was analysed by repeated measures analysis of variance (ANOVA) of log-transformed data.

RESULTS

At baseline, women with higher VAT had significantly lower CTX-1 and higher PTH (44% lower and 30% higher, respectively between Q4 and Q1, P < 0.0001). VAT had no influence on the acute changes in CTX-1 or PTH with calcium or juice. A suppression of 44% in CTX-1 was seen with calcium carbonate and milk and a suppression of 18% with juice. PTH was suppressed more with calcium carbonate (47%) compared to milk (22%). Milk calcium reduced PTH and CTX-1 at 2-hour while calcium carbonate reduced PTH in 1 hour. The suppression in CTX-1 was slower with lowest levels at 4-5 hour.

CONCLUSIONS

1000 mg calcium obtained from milk or from calcium carbonate is effective in acutely suppressing bone resorption in postmenopausal women irrespective of visceral fat.This trial is registered with the Australian New Zealand Clinical Trials Registry http://www.ANZCTR.org.au/ACTRN12617000779370.aspx (ACTRN 12617000779370).

Effect of Dipeptidyl Peptidase-4 Inhibitors on Bone Health in Patients with Type 2 Diabetes Mellitus

Patients with type 2 diabetes (T2DM) have a higher risk of bone fracture even when bone mineral density (BMD) values are normal. The trabecular bone score (TBS) was recently developed and used for evaluating bone strength in various diseases. We investigated the effect of DPP-4 inhibitors on bone health using TBS in patients with T2DM. This was a single-center, retrospective case-control study of 200 patients with T2DM. Patients were divided into two groups according to whether they were administered a DPP-4 inhibitor (DPP-4 inhibitor group vs. control group). Parameters related to bone health, including BMD, TBS, and serum markers of calcium homeostasis, were assessed at baseline and after one year of treatment. We found TBS values increased in the DPP-4 group and decreased in the control, indicating a significant difference in delta change between them. The BMD increased in both groups, with no significant differences in delta change between the two groups observed. Serum calcium and 25-hydroxy vitamin D3 increased only in the DPP-4 inhibitor group, while other glycemic parameters did not show significant differences between the two groups. Treatment with DPP-4 inhibitors was associated with favorable effects on bone health evaluated by TBS in patients with T2DM.

Analysis of Body Perception, Preworkout Meal Habits and Bone Resorption in Child Gymnasts

The beneficial effects of physical activity on body image perception and bone are debated among artistic gymnasts. Gymnasts seem to be at greater risk of developing body dissatisfaction, eating disorders and osteoporosis due to inadequate nutrition and attention to the appearance of the body. The objective of this work was to investigate the association between the artistic gymnast and a more favorable body image compared to their sedentary peers and if a preworkout high-carbohydrate meal (HCM; 300 kcal, 88% carbohydrates, 9% protein, 3% fat) or high-protein meal (HPM; 300 kcal, 55% carbohydrates, 31% protein, 13% fat) is able to attenuate bone resorption in young rhythmic gymnasts. Twenty-eight preadolescent female gymnasts were examined. Self-esteem tests were used to analyze body image perception. Preworkout eating habits were examined by short food frequency questions (FFQ) validated for children. The biomarker of the bone resorption C-terminal telopeptide region of collagen type 1 (CTX) was measured in the urine (fasting, postmeal and postworkout). Gymnasts reported higher satisfaction with their body appearance compared to sedentary peers. Of the gymnasts, 30% did not have a preworkout meal regularly, and the timing of the consumption was variable. Bone resorption was decreased by the HCM, consumed 90 min before the training, with respect to the HPM. The study suggests that playing artistic gymnastics is associated with a positive body self-perception in a child. The variability in preworkout meal frequency and timing need attention to prevent inadequate eating habits in light of the ability of the HCM to reduce acute bone resorption.

Consumption of nutrients and insulin resistance suppress markers of bone turnover in subjects with abdominal obesity

OBJECTIVE

Abdominal obesity and type 2 diabetes are associated with insulin resistance and low bone turnover along with an increased fracture risk. The mode of action is poorly understood. The bone resorption marker, C-terminal telopeptide type 1 collagen (CTX), and to a lesser extent, the bone formation marker, Procollagen type 1 N-terminal propeptide (P1NP) appear to be inhibited by food consumption. The link between food consumption, insulin resistance and bone turnover remains to be clarified. Primarily we aimed to compare the postprandial CTX, P1NP and PTH responses by two frequently applied methods in assessing metabolic health; oral glucose tolerance test (OGTT) and mixed meal tolerance test. Secondly, we explored the effect of insulin resistance on bone marker responses.

METHODS

We enrolled 64 subjects with abdominal obesity. Following 10 h of fasting, subjects initially underwent a standard OGTT (300 kcal) and approximately one week later a mixed meal tolerance test (1130 kcal). Circulating CTX, P1NP and PTH were assessed on both days at time = 0, after 30 min and after 90 min for comparison of the two interventions. We analyzed glucose and insulin levels for the assessment of insulin resistance. Additionally, we measured plasma calcium levels along with the gut hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like-peptide 2 (GLP-2) in an attempt to identify possible mediators of the postprandial bone response.

RESULTS

CTX, P1NP and PTH were suppressed by OGTT and the mixed meal; the latter induced a more pronounced suppression after 90 min. Calcium levels were similar between OGTT and meal. GIP and GLP-2 levels increased after both interventions, although only the meal induced a sustained increase after 90 min. Fasting P1NP was inversely associated with insulin resistance. The meal-induced suppression of P1NP (but not CTX or PTH) was inversely associated with level of insulin resistance.

CONCLUSION

The acute postprandial suppression of bone turnover markers is extended after ingestion of a mixed meal compared to an OGTT. The response appears to be independent of gender and prompted by a reduction in PTH. The study additionally indicates a possible link between the development of insulin resistance and low bone turnover - which may be of key essence in the development of the fragile bone structure and increased fracture risk demonstrated in subjects with abdominal obesity and T2D.

GIP's involvement in the pathophysiology of type 2 diabetes

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

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

CONTEXT

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

OBJECTIVE

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

DESIGN

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

PATIENTS

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

INTERVENTIONS

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

MAIN OUTCOME MEASURES

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

RESULTS

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

CONCLUSIONS

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

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.

Lower Bone Mass and Higher Bone Resorption in Pheochromocytoma: Importance of Sympathetic Activity on Human Bone

CONTEXT

Despite the apparent biological importance of sympathetic activity on bone metabolism in rodents, its role in humans remains questionable.

OBJECTIVE

To clarify the link between the sympathetic nervous system and the skeleton in humans.

DESIGN, SETTING, AND PATIENTS

Among 620 consecutive subjects with newly diagnosed adrenal incidentaloma, 31 patients with histologically confirmed pheochromocytoma (a catecholamine-secreting neuroendocrine tumor) and 280 patients with nonfunctional adrenal incidentaloma were defined as cases and controls, respectively.

RESULTS

After adjustment for confounders, subjects with pheochromocytoma had 7.2% lower bone mass at the lumbar spine and 33.5% higher serum C-terminal telopeptide of type 1 collagen (CTX) than those without pheochromocytoma (P = 0.016 and 0.001, respectively), whereas there were no statistical differences between groups in bone mineral density (BMD) at the femur neck and total hip and in serum bone-specific alkaline phosphatase (BSALP) level. The odds ratio (OR) for lower BMD at the lumbar spine in the presence of pheochromocytoma was 3.31 (95% confidence interval, 1.23 to 8.56). However, the ORs for lower BMD at the femur neck and total hip did not differ according to the presence of pheochromocytoma. Serum CTX level decreased by 35.2% after adrenalectomy in patients with pheochromocytoma, whereas serum BSALP level did not change significantly.

CONCLUSIONS

This study provides clinical evidence showing that sympathetic overstimulation in pheochromocytoma can contribute to adverse effects on human bone through the increase of bone loss (especially in trabecular bone), as well as bone resorption.

Dipeptidyl Peptidase-4 and Adolescent Idiopathic Scoliosis: Expression in Osteoblasts

It has been proposed that girls with adolescent idiopathic scoliosis (AIS) tend to have a taller stature and a lower body mass index. Energy homeostasis, that is known to affect bone growth, could contribute to these characteristics. In circulation, dipeptidyl peptidase-4 (DPP-4) inactivates glucagon-like peptide-1 (GLP-1), an incretin that promotes insulin secretion and sensitivity. Our objectives were to investigate DPP-4 status in plasma and in osteoblasts of AIS subjects and controls and to evaluate the regulatory role of metabolic effectors on DPP-4 expression. DPP-4 activity was assessed in plasma of 113 girls and 62 age-matched controls. Osteoblasts were isolated from bone specimens of AIS patients and controls. Human cells were incubated with glucose, insulin, GLP-1 and butyrate. Gene and protein expressions were evaluated by RT-qPCR and Western blot. Our results showed 14% inferior plasma DPP-4 activity in AIS patients when compared to healthy controls (P = 0.0357). Similarly, osteoblasts derived from AIS subjects had lower DPP-4 gene and protein expression than controls by 90.5% and 57.1% respectively (P < 0.009). DPP-4 expression was regulated in a different manner in osteoblasts isolated from AIS participants compared to controls. Our results suggest a role for incretins in AIS development and severity.

Obesity is a concern for bone health with aging

Accumulating evidence supports a complex relationship between adiposity and osteoporosis in overweight/obese individuals, with local interactions and endocrine regulation by adipose tissue on bone metabolism and fracture risk in elderly populations. This review was conducted to summarize existing evidence to test the hypothesis that obesity is a risk factor for bone health in aging individuals. Mechanisms by which obesity adversely affects bone health are believed to be multiple, such as an alteration of bone-regulating hormones, inflammation, oxidative stress, the endocannabinoid system, that affect bone cell metabolism are discussed. In addition, evidence on the effect of fat mass and distribution on bone mass and quality is reviewed together with findings relating energy and fat intake with bone health. In summary, studies indicate that the positive effects of body weight on bone mineral density cannot counteract the detrimental effects of obesity on bone quality. However, the exact mechanism underlying bone deterioration in the obese is not clear yet and further research is required to elucidate the effect of adipose depots on bone and fracture risk in the obese population.

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.

The impact of type 2 diabetes on bone metabolism

Diabetes complications and osteoporotic fractures are two of the most important causes of morbidity and mortality in older patients and 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 quality. Both glycemic and bone homeostasis are under control of common regulatory factors. These factors include insulin, accumulation of advanced glycation end products, peroxisome proliferator-activated receptor gamma, gastrointestinal hormones (such as the glucose-dependent insulinotropic peptide and the glucagon-like peptides 1 and 2), and bone-derived hormone osteocalcin. This background allows individual pharmacological targets for antidiabetic therapies to affect the bone quality due to their indirect effects on bone cell differentiation and bone remodeling process. Moreover, it's important to consider the fragility fractures as another diabetes complication and discuss more deeply about the requirement for adequate screening and preventive measures. This review aims to briefly explore the impact of T2DM on bone metabolic and mechanical proprieties and fracture risk.

Significant inverse association between serum osteocalcin and incident type 2 diabetes in a middle-aged cohort

BACKGROUND

Accumulating evidence indicates that osteocalcin links bone formation to glucose homeostasis. However, the correlation between osteocalcin and incident type 2 diabetes has been controversial based on the limited results of cohort studies. We examined the link between serum osteocalcin and glucose homeostasis including incident type 2 diabetes in a 3-year follow-up study.

METHODS

This retrospective study enrolled 1870 middle-aged subjects (1279 men, 591 women) at Chinese PLA General Hospital who were followed-up for 3 years. Cox proportional hazards regression was used to determine whether incident type 2 diabetes was influenced by the osteocalcin concentrations measured with an electrochemiluminescence immunoassay.

RESULTS

At baseline, the blood glucose levels and prevalence of metabolic syndrome varied inversely with the osteocalcin quartiles. During follow-up, type 2 diabetes developed in 80 of the 1870 subjects. The prevalence decreased with osteocalcin quartiles (P = 0.016). In models adjusted for metabolism-related parameters, osteocalcin was inversely associated with fasting plasma glucose {β = -0.017 [95% confidence interval (CI), -0.034-0.00], P = 0.040}. Osteocalcin was inversely related to the risk of incident type 2 diabetes assessed using a model adjusted for glucose metabolic parameters, 25-hydroxy vitamin D3 and parathyroid hormone (hazard ratio [HR] = 0.09 [95% CI, 0.01-0.96], P = 0.046). The onset risk of diabetes in the first osteocalcin quartile was higher than in the fourth quartile (HR = 1.67 [95% CI, 0.96-3.48], P = 0.035). The correlation was strongly significant after fully adjusting for glucose related parameters and bone turnover (HR = 3.02 [95% CI, 1.25-7.32], P = 0.014).

CONCLUSIONS

Low serum osteocalcin concentrations at baseline were independently related to an increased risk of incident type 2 diabetes. Copyright © 2016 John Wiley & Sons, Ltd.

Risk of fractures and diabetes medications: a nationwide cohort study

The effects of diabetes medications on risk of fracture were investigated using the South Korea nationwide claims database. We demonstrated that the use of dipeptidyl peptidase-4 inhibitor could be associated with decreased risk of fracture. Thiazolidinedione use was associated with about 60 % increased risk of fracture in real clinical practice.

INTRODUCTION

The effects of diabetes medication on fracture have important clinical health consequences, since most diabetes patients are at high risk of fracture. We aimed to investigate the effect of diabetes medication on fracture risk.

METHODS

The nationwide medical claim database in South Korea was investigated. Among 2,886,555 subjects with antidiabetes prescriptions, 207,558 subjects aged 50 years and older, who initiated diabetes medication from 2008 to 2011, were analyzed. The subjects were classified based on diabetes medication classes: non-user (insufficient exposure), metformin (MET), sulfonylurea (SU), alpha-glucosidase inhibitor (AGI), MET + SU, MET + thiazolidinedione (TZD), MET + dipeptidyl peptidase-4 inhibitor (DPP4-I), and SU + TZD.

RESULTS

A total of 5996 fractures were observed. The fracture rate varied significantly across type of diabetes medications, with MET + DPP4-I combination group having the lowest rate and SU + TZD combination group having the highest rate. Compared to non-users, MET + DPP4-I inhibitor combination group had significantly reduced composite fracture risk (hazard ratio (HR) = 0.83, P = 0.025) and significantly reduced vertebral fracture risk (HR = 0.73, P = 0.013) in the unadjusted analysis. Compared to MET + SU users, MET + DPP4-I users showed a trend of lower non-vertebral fracture risk (HR = 0.82, P = 0.086) after adjusting for all confounding variables. Patients using TZD had significantly increased risk of fracture (HR = 1.59, P < 0.001) compared with patients not using TZDs adjusting for all confounding variables.

CONCLUSIONS

The results of this nationwide study showed a trend that DPP4 inhibitor might have a protective effect on bone metabolism compared with SU, when added to MET. Clinicians should take these results into consideration when prescribing diabetes medication, especially in elderly patients or those at high risk or fracture.

Protein/amino-acid modulation of bone cell function

Nutrients (protein, carbohydrates and fats) have traditionally been thought of as fuels simply providing the energy for cellular metabolic activity. According to the classic view, if nutrients are available, then anabolic pathways are activated, and if nutrients are not available, catabolic pathways are activated. However, it is becoming increasingly clear that nutrient effects on bone cells (stem cells, osteoblasts and osteoclasts) are complex, some nutrients promote bone formation, whereas others interfere with bone formation or actually promote bone break down. At an organ level, nutrient intake can suppress bone breakdown and modulate the activity of the calcium/vitamin D/parathyroid hormone axis. At a cellular level, nutrient intake can impact cellular energetics either through a direct mechanism (binding or uptake of the nutrient into the cell) or indirect (by elevating nutrient-related hormones such as insulin, insulin-like growth factor 1 or incretin hormones). It is also becoming clear that within a nutrient class (for example, protein), individual components (that is, amino acids) can have markedly different effects on cell function and impact bone formation. The focus of this review will be on one nutrient class in particular, dietary protein. As the prevalence of inadequate dietary protein intake increases with age, these findings may have translational implications as to the optimal dietary protein content in the setting of age-associated bone loss.

Type 2 diabetes and the skeleton: new insights into sweet bones

Substantial evidence shows that skeletal fragility should be considered among the complications associated with type 2 diabetes. Individuals with type 2 diabetes have increased fracture risk, despite normal bone mineral density (BMD) and high BMI-factors that are generally protective against fractures. The mechanisms underlying skeletal fragility in diabetes are not completely understood, but are multifactorial and likely include effects of obesity, hyperglycaemia, oxidative stress, and accumulation of advanced glycation end products, leading to altered bone metabolism, structure, and strength. Clinicians should be aware that BMD measurements underestimate fracture risk in people with type 2 diabetes, and that new treatments for diabetes, with neutral or positive effects on skeletal health, might play a part in the management of diabetes in those at high risk of fracture. Data for the optimum management of osteoporosis in patients with type 2 diabetes are scarce, but in the absence of evidence to the contrary, physicians should follow guidelines established for postmenopausal osteoporosis.

An oral cathepsin K inhibitor ONO-5334 inhibits N-terminal and C-terminal collagen crosslinks in serum and urine at similar plasma concentrations in postmenopausal women

Relationships between the plasma concentration of a cathepsin K inhibitor (ONO-5334) and inhibition of bone resorption markers N-telopeptide of type I collagen (NTX) and C-telopeptide of type I collagen (CTX) in serum and urinary NTX/creatinine and CTX/creatinine were examined in 10 postmenopausal women. The subjects received slow-release tablets of 100mg ONO-5534 under fasted or fed conditions in a study with a crossover design. Inhibition of serum NTX and CTX levels and plasma concentrations of ONO-5334 were monitored at 0, 24, 48 and 168 h after dosing. Changes in urinary NTX/creatinine and CTX/creatinine levels in second morning urine were evaluated on 0, 1, 2 and 7 days after dosing. Data were analyzed using sigmoid maximal drug effect (Emax) models. The maximal inhibition, estimated Emax values, were -31.8% for serum NTX, -53.1% for serum CTX, -67.2% for urinary NTX/creatinine, and -95.2% for urinary CTX/creatinine. The estimated half maximal effective plasma concentrations (EC50) of ONO-5334 and confidence intervals were 1.79 (1.01 to 3.16) ng/mL for serum NTX, 2.07 (1.63 to 2.62) ng/mL for serum CTX, 1.85 (1.30 to 2.61) ng/mL for urinary NTX/creatinine, and 1.98 (0.94 to 3.76) ng/mL for urinary CTX/creatinine. EC50 values for the four crosslinks did not significantly differ, as indicated by the overlapping 95% confidence intervals. The highest signal-to-noise ratio was achieved with serum CTX, and was 2-fold higher than that on serum NTX. Inhibition for serum NTX and CTX, and urinary NTX/creatinine and CTX/creatinine by ONO-5334 were all correlated with correlation coefficients ranging from 0.55 to 0.80. In conclusion, data of ONO-5334 slow-releasing tablets in postmenopausal women were well fitted in Emax model. In all measured telopeptides, the maximal inhibition was obtained at urinary CTX/creatinine level, but serum CTX had the highest signal-to-noise ratio. Inhibition for all measured telopeptides by ONO-5334 were all correlated. The estimated half maximal effective plasma concentrations were not significantly different between all measured telopeptides.

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.

Antidiabetic therapy effects on bone metabolism and fracture risk

Patients with diabetes are at greater risk of fractures mostly due to not only to extraskeletal factors, such as propensity to fall, but also to bone quality alteration, which reduces bone strength. In people with diabetes, insulin deficiency and hyperglycaemia seem to play a role in determining bone formation alteration by advanced glycation end product (AGE) accumulation or AGE/RAGE (receptors for AGE) axis imbalance, which directly influence osteoblast activity. Moreover, hyperglycaemia and oxidative stress are able to negatively influence osteocalcin production and the Wnt signalling pathways with an imbalance of osteoblast/osteoclast activity leading to bone quality reduction as global effect. In addition, other factors such as insulin growth factors and peroxisome proliferator-activated receptor-γ pathways seem to have an important role in the pathophysiology of osteoporosis in diabetes. Although there are conflicting data in literature, adequate glycaemic control with hypoglycaemic treatment may be an important element in preventing bone tissue alterations in both type 1 and type 2 diabetes. Attention should be paid to the use of thiazolidinediones, especially in older women, because the direct negative effect on bone could exceed the positive effect of glycaemic control. Finally, preliminary data on animals and in humans suggest the hypothesis that incretins and dipeptidyl peptidase-4 inhibitors could have a positive effect on bone metabolism by a direct effect on bone cells; however, such issue needs further investigations.

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.

An association between post-meal bile acid response and bone resorption in normal subjects

Background

The mechanism surrounding bone suppression after a meal may involve several mediators, but is yet to be clarified. Bile acids (BA) function as signalling molecules in response to feeding, and may be directly involved in bone suppression acutely after a meal. The aim of this study was to test the hypothesis that BA are involved in the acute bone suppression observed after a meal.

Methods

A prospective study in which samples collected from volunteers fed a 400 Kcal test meal after an overnight fast were analysed for parathyroid hormone (PTH), BA, and carboxyterminal of type 1 collagen telopeptide (CTX). The study was carried out in 10 healthy male volunteers. Ethical approval was obtained from the Local Research and Ethics Committee at King's College Hospital.

Results

Total BA, glycine conjugated bile acids (GCBA), PTH and CTX showed a response to meal ingestion. There was a negative correlation between percentage change in PTH and CTX ( R2 = −0.82, P = 0.004), and between PTH and GCBA ( R2 = −0.39, P = 0.005).

Conclusion

This study demonstrated an association between GCBA and PTH suppression after a meal. The drop in PTH concentration after a meal may be responsible for the suppression of bone resorption as observed by the decrease in CTX concentration.

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.

Hyperinsulinemia and urinary calcium excretion in calcium stone formers with idiopathic hypercalciuria

CONTEXT

Calcium stone formers with idiopathic hypercalciuria (IH) are known to exhibit an exaggerated postprandial rise in urine calcium excretion compared with non-stone-forming individuals, and insulin has been proposed to mediate this difference.

OBJECTIVE

Our objective was to investigate the impact of hyperinsulinemia on urine calcium excretion in IH compared with non-stone-forming controls.

PARTICIPANTS AND SETTING

Ten IH patients and 22 control non-stone-forming subjects (8 lean and 14 overweight and obese) participated at the University of Texas Southwestern Clinical and Translational Research Center.

DESIGN

After stabilization on a fixed metabolic diet, subjects underwent a hyperinsulinemic-euglycemic clamp. Fasting 2-hour urine specimens were collected before and during the clamp.

MAIN OUTCOME MEASURES

Changes in fractional calcium excretion (F(E)Ca) during the clamp were compared between the 3 groups of subjects (IH, overweight/obese controls, and lean controls). Insulin sensitivity was measured by glucose disposal rate.

RESULTS

IH had significantly higher 24-hour urine calcium excretion than controls, and exhibited similar age, body mass index, and insulin sensitivity as overweight/obese controls. The hyperinsulinemic-euglycemic clamp resulted in a significant increase in serum insulin with no significant changes in serum calcium and glucose. This was accompanied by a small increase in F(E)Ca, with no significant differences between the 3 groups. There was no correlation between insulin sensitivity and 24-hour urine calcium or the change in F(E)Ca during the hyperinsulinemic clamp.

CONCLUSIONS

The rise in urine calcium associated with euglycemic hyperinsulinemia was small and not statistically different between IH and non-stone-forming controls. Insulin is therefore unlikely to play a significant pathogenetic role in IH.

Amylin exerts osteogenic actions with different efficacy depending on the diabetic status

Amylin displays osteogenic features, but its role in diabetic osteopenia is unclear. We examined the possible osteogenic action of amylin infusion for 3days into fructose-induced insulin-resistant (IR) and streptozotocin-induced type 2 diabetic (T2D) and normal (N) rats. Amylin failed to affect glycaemia or parathyroid hormone levels in any group, but reduced hyperinsulinemia in IR rats. In N rats, amylin increased bone formation rate and reduced osteoclast surface and erosive surface in the femoral metaphysis, and increased osteoprotegerin (OPG)/receptor activator of NFκB ligand (RANKL) mRNA ratio in the tibia. In T2D rats, amylin normalized trabecular structure parameters and increased osteoblast number and osteocalcin (OC) expression in long bones. In contrast, in IR rats, no apparent osteogenic effect of amylin in the femur was observed, although both OC and OPG/RANKL ratio were increased in the tibia. Our findings demonstrate a different osteogenic efficacy of amylin in two diabetic settings.

The role of osteocalcin in human glucose metabolism: marker or mediator?

Increasing evidence supports an association between the skeleton and energy metabolism. These interactions are mediated by a variety of hormones, cytokines and nutrients. Here, the evidence for a role of osteocalcin in the regulation of glucose metabolism in humans is reviewed. Osteocalcin is a bone matrix protein that regulates hydroxyapatite size and shape through its vitamin-K-dependent, γ-carboxylated form. The concentration of osteocalcin in the circulation is a measure of bone formation. The undercarboxylated form of osteocalcin is active in glucose metabolism in mice. Total serum osteocalcin concentrations in humans are inversely associated with measures of glucose metabolism; however, human data are inconclusive with regard to the role of uncarboxylated osteocalcin in glucose metabolism because most studies do not account for the influence of vitamin K on the proportion of undercarboxylated osteocalcin or differentiate between the total and uncarboxylated forms of osteocalcin. Furthermore, most human studies do not concomitantly measure other bone turnover markers to isolate the role of osteocalcin as a measure of bone formation from its effect on glucose metabolism. Carefully designed studies are required to define the role of osteocalcin and its carboxylated or undercarboxylated forms in the regulation of glucose metabolism in humans.

Bone metabolism in obesity and weight loss

Excess body weight due to obesity has traditionally been considered to have a positive effect on bone; however, more recent findings suggest that bone quality is compromised. Both obesity and caloric restriction increase fracture risk and are regulated by endocrine factors and cytokines that have direct and indirect effects on bone and calcium absorption. Weight reduction will decrease bone mass and mineral density, but this varies by the individual's age, gender, and adiposity. Dietary modifications, exercise, and medications have been shown to attenuate the bone loss associated with weight reduction. Future obesity and weight loss trials would benefit from assessment of key hormones, adipokine and gut peptides that regulate calcium absorption, and bone mineral density and quality by using sensitive techniques in high-risk populations.

Bone: incretin hormones perceiver or receiver?

Novel incretin-based drugs, such as glucagon-like peptide-1 receptor agonists (GLP-1 RA) and dipeptidyl peptidase-4 inhibitors (DPP-4i), have been last introduced in the pharmacological treatment of type 2 diabetes. In the last few years, the interest on the relationship of gut hormones with bone metabolism in diabetes has been increasing. The aim of present paper is to examine in vitro and in vivo evidence on the connections between incretin hormones and bone metabolism. We also discuss results of clinical trials and metaanalysis, explore the effects of incretin drugs in vitro on osteogenic cells and osteoclasts, and speculate on the possibility of different effects of GLP-1 RA and DPP-4i on the risk of bone fractures risk in humans. Although existing preliminary evidence suggests a protective effect on the bone, at least for DPP-4i, further controlled, long-term studies with measurement of bone markers, bone density, and clinical fractures rates are needed to substantiate and confirm those findings.

Effects of vildagliptin on postprandial markers of bone resorption and calcium homeostasis in recently diagnosed, well-controlled type 2 diabetes patients

BACKGROUND

Bone metabolism is a dynamic process that is influenced by food ingestion. Endogenous incretins have been shown to be important regulators of bone turnover. The aim of the present study was to assess whether a dipeptidylpeptidase (DPP)-4 inhibitor affects markers of bone resorption and calcium homeostasis.

METHODS

The present study was a single-center, double blind, randomized clinical trail. Fifty-nine drug-naïve patients with type 2 diabetes (T2D) were randomized to either 1 year treatment with the DPP-4 inhibitor vildagliptin (100 mg, once daily; n = 29) or placebo (n = 30). Patients received a standardized breakfast after measurement of serum concentrations of cross-linked C-terminal telopeptide (s-CTx), a bone resorption marker influenced by food intake, before and after 50 weeks treatment.

RESULTS

Vildagliptin did not change postprandial s-CTx concentrations compared with pretreatment levels (between-group ratio 1.15 ± 0.17; P = 0.320). Fasting serum alkaline phosphatase, calcium, and phosphate were also unaffected y 1 year treatment with vildagliptin.

CONCLUSIONS

Treatment with vildagliptin for 1 year was not associated with changes in markers of bone resorption and calcium homeostasis in drug-naïve patients with T2D and mild hyperglycemia.

Clinical use of bone turnover markers to monitor pharmacologic fracture prevention therapy

Monitoring of drug therapies to prevent fractures is controversial. Measurement of bone turnover markers has the potential to identify those with a suboptimal response to fracture prevention medication within a few months of its commencement. However, given the imprecision of currently commercially available assays of bone turnover markers, many individual persons who are “suboptimal medication responders” are likely to be misclassified as “adequate responders” or vice versa, depending on the cut point chosen to define suboptimal and adequate response. Before bone turnover markers can be recommended for routine use in clinical practice to monitor fracture prevention therapies, three advances are needed: 1) bone marker assays with better precision; 2) research establishing optimal cut points of bone marker levels to distinguish “suboptimal responders” from “adequate responders”; and 3) research establishing the incremental fracture reduction benefit from clinical interventions for “suboptimal responders” identified from bone marker measurements.

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.

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.

The roles of the sympathetic nervous system in osteoporotic diseases: A review of experimental and clinical studies

With the rapid aging of the world population, the issue of skeletal health is becoming more prominent and urgent. The bone remodeling mechanism has sparked great interest among bone research societies. At the same time, increasing clinical and experimental evidence has driven attention towards the pivotal role of the sympathetic nervous system (SNS) in bone remodeling. Bone remodeling is thought to be partially controlled by the hypothalamus, a process which is mediated by the adrenergic nerves and neurotransmitters. Currently, new knowledge about the role of the SNS in the development and pathophysiology of osteoporosis is being generated. The aim of this review is to summarize the evidence that proves the involvement of the SNS in bone metabolism and to outline some common osteoporotic diseases that occur under different circumstances. The adrenergic signaling pathway and its neurotransmitters are involved to various degrees of importance in the development of osteoporosis in postmenopause, as well as in spinal cord injury, depression, unloading and the complex regional pain syndrome. In addition, clinical and pharmacological studies have helped to increase the comprehension of the adrenergic signaling pathway. We try to individually examine the contributions of the SNS in osteoporotic diseases from a different perspective. It is our hope that a further understanding of the adrenergic signaling by the SNS will pave the way for conceptualizing optimal treatment regimens for osteoporosis in the near future.

Exenatide treatment did not affect bone mineral density despite body weight reduction in patients with type 2 diabetes

Preclinical studies suggest that incretin-based therapies may be beneficial for the bone; however, clinical data are largely lacking. We assessed whether the differential effects of these therapies on body weight differed with respect to their effect on bone mineral density (BMD) and markers of calcium homeostasis in patients with type 2 diabetes (T2D). Sixty-nine metformin-treated patients with T2D were randomized to exenatide twice daily (n = 36) or insulin glargine once daily (n = 33). Total body BMD, measured by dual-energy X-ray absorptiometry, and serum markers of calcium homeostasis were assessed before and after 44-week treatment. Exenatide or insulin glargine treatment decreased body weight by 6%. Endpoint BMD was similar in both groups after 44-week therapy (LSmean ± s.e.m. between-group difference -0.002 ± 0.007 g/cm(2) ; p = 0.782). Fasting serum alkaline phosphatase, calcium and phosphate remained unaffected. Forty-four-week treatment with exenatide or insulin glargine had no adverse effects on bone density in patients with T2D, despite differential effects on body weight.

Osteoporosis and risk of fracture in patients with diabetes: an update

Diabetes mellitus (DM) and osteoporotic fractures are two of the most important causes of mortality and morbidity in older subjects. Recent data report a close association between fragility fracture risk and DM of both type 1 (DM1) and type 2 (DM2). However, DM1 is associated with reduced bone mineral density (BMD), whereas patients with DM2 generally have normal or increased BMD. This apparent paradox may be explained by the fact that, at a given level of BMD, diabetic patients present lower bone quality with respect to non-diabetics, as shown by several studies reporting that diabetes may affect bone tissue by means of various mechanisms, including hyperinsulinemia, deposition of advanced glycosylation endproducts (AGEs) in collagen, reduced serum levels of IGF-1, hypercalciuria, renal failure, microangiopathy and inflammation. In addition, the propensity to fall and several comorbidities may further explain the higher fracture incidence in DM patients with respect to the general population. It is reasonable to expect that close metabolic control of diabetes may improve bone status, although its effect on reduction of fracture risk has not yet been demonstrated. However, metformin has a direct effect on bone tissue by reducing AGE accumulation, whereas insulin acts directly on osteoclast activity, and thiazolidinediones (TZD) may have a negative effect by switching mesenchymal progenitor cells to adipose rather than bone tissue. New prospects include the incretins, a class of antidiabetic drugs which may play a role linking nutrition and bone metabolism. Better knowledge on how diabetes and its treatments influence bone tissue may lie at the basis of effective prevention of bone fracture in diabetic patients. Thus, close glycemic control, adequate intake of calcium and vitamin D, screening for low BMD, and prevention and treatment of diabetic complications are key elements in the management of osteoporosis in both DM1 and DM2. Attention should be paid to treating diabetes with TZD in women with DM2, particularly if elderly. Lastly, patients with osteoporosis and diabetes should be offered the same pharmacological treatments as non-diabetics, although specific trials on the effects of anti-osteoporotic drugs in the diabetic population are lacking.

Mechanisms of impaired bone strength in type 1 and 2 diabetes

Diabetes and osteoporosis are common and complex disorders with an enormous health burden that can be often associated especially in middle-age and elderly individuals. Although there is raising awareness of the higher fractures rates among patients with type 1 (DM1) and 2 (DM2) diabetes, there are few data available on the pathogenetic mechanisms responsible for this increased risk. Importantly, several experimental and clinical observations suggest that bone abnormalities associated with diabetes may differ, at least in part, from those associated with senile or post-menopausal osteoporosis. This implies that specific preventive and therapeutic strategies have to be developed and tested to prevent fractures in DM1 and DM2 patients. It is also likely that shared (i.e. due to glucose-toxicity) as well as different (i.e. due to insulin levels or other hormones) mechanisms may be associated with bone fragility in DM1 and DM2. Moreover, the hypothesis of an endocrine role of the skeleton in the regulation of glucose metabolism and insulin sensitivity has been recently proposed by experimental observations. This review summarizes the recent clinical and experimental advances on glucose tolerance, bone fragility and osteoporosis associated with diabetes.

Presence of a functional receptor for GLP-1 in osteoblastic cells, independent of the cAMP-linked GLP-1 receptor

Glucagon-like peptide 1 (GLP-1) controls glucose metabolism in extrapancreatic tissues through receptors other than the pancreatic cAMP-linked GLP-1 receptor; also, GLP-1 induces an insulin- and PTH-independent bone anabolic action in insulin-resistant and type-2 diabetic rats. Here we searched for the presence and characteristics of GLP-1 receptors in osteoblastic MC3T3-E1 cells. [(125)I]-GLP-1 specific binding to MC3T3-E1 cells was time- and temperature-dependent, reaching maximal value at 30 min at 25 degrees C; in these conditions, [(125)I]-GLP-1 binding was dissociable, and displaced by GLP-1, partially by GLP-2, but not by exendin-4 (Ex-4), exendin-9 (Ex-9), glucagon or insulin; Scatchard analysis of the unlabeled GLP-1 data showed high and low affinity binding sites; cross-linking of GLP-1 binding revealed an estimated 70 kDa band, almost undetectable in the presence of 10(-6) M GLP-1. GLP-1, Ex-9, insulin or glucagon failed to modify cellular cAMP content, while GLP-2 and Ex-4 increased it. However, GLP-1 induced an immediate hydrolysis of glycosylphosphatidylinositols (GPIs) generating short-lived inositolphosphoglycans (IPGs), and an increase in phosphatidylinositol-3 kinase (PI3K) and mitogen activated protein kinase (MAPK) activities; Ex-4 also affected GPIs, but its action was delayed with respect to that of GLP-1. This incretin was found to decrease Runx2 but increased osteocalcin gene expression, without affecting that of osteoprotegerin or the canonical Wnt pathway activity in MC3T3-E1 cells which do not express the pancreatic GLP-1 receptor. Our data demonstrate for the first time that GLP-1 can directly and functionally interact with osteoblastic cells, possibly through a GPI/IPG-coupled receptor.

ACTH is a novel regulator of bone mass

Adrenocorticotropin (ACTH) is one of several peptide hormones derived from a larger molecule, proopiomelanocortin (POMC). ACTH is a classic endocrine hormone, processed and secreted from the pituitary to stimulate cortisol production from the fasciculata cells in the adrenal gland. However, ACTH is also produced by other cells, including macrophages, at many sites in the body. ACTH binds to a specific member of the melanocortin receptor family, the MC2R. MC2R is expressed in osteoblastic cells in vivo, as shown by in situ hybridization. MC2R expression is strongest at sites of active bone deposition, and thus ACTH response probably varies with osteoblastic activity or stage of osteoblast differentiation. In vitro ACTH stimulates proliferation of osteoblasts in a dose-dependent manner. ACTH at 10 nM increases collagen I mRNA in the osteoblastic cell line SaOs2, although at lower concentrations ACTH may oppose osteoblast differentiation. ACTH is thus, at high concentrations, anabolic for the osteoblast, and it is highly likely that the hormone has concentration-dependent effects on bone metabolism in vivo.

Reduced amylin levels are associated with low bone mineral density in women with anorexia nervosa

CONTEXT

Anorexia nervosa, characterized by extreme low body weight due to reduced nutrient intake, is associated with severe bone loss. Peptide hormones, including amylin, GIP, and GLP2, are released immediately after nutrient intake and may be involved in the regulation of bone turnover.

OBJECTIVE

To investigate fasting levels of amylin, GIP, and GLP2 and their relationships with bone mineral density (BMD) in women with anorexia nervosa compared to healthy controls.

DESIGN

Cross-sectional.

SETTING

Clinical Research Center.

STUDY PARTICIPANTS

15 women with anorexia nervosa and 16 healthy controls.

INTERVENTION

None.

MAIN OUTCOME MEASURES

Fasting serum amylin, GIP, and GLP2, and BMD.

RESULTS

Women with anorexia nervosa had significantly lower fasting serum amylin and GIP levels than healthy controls. Fasting serum GLP2 levels were not significantly different between groups. Fasting amylin levels were positively associated with BMD and Z-score at the PA spine, total hip, and femoral neck. Fasting amylin levels were also positively associated with weight and percent fat; after controlling for these variables, amylin was still a significant predictor of BMD and Z-score at the femoral neck and of Z-score at the total hip. In the anorexia nervosa group, there was a trend toward an inverse association between amylin and C-terminal telopeptide (CTX) levels (R=-0.47, p=0.08). GIP and GLP2 levels did not predict BMD at any site.

CONCLUSION

Decreased secretion of amylin may be a mechanism through which reduced nutrient intake adversely affects BMD in anorexia nervosa.

Exendin-4 exerts osteogenic actions in insulin-resistant and type 2 diabetic states

Poor control of glucose homeostasis accounts for diabetes-related bone loss. Incretins - GLP-1 and GIP - have been proposed to affect bone turnover. GLP-1, apart from its anti-diabetic and other actions, has shown to exert a bone anabolic effect in streptozotocin-induced type 2 diabetic (T2D) and fructose-induced insulin-resistant (IR) rats. Exendin-4 (Ex-4), a peptide of non-mammalian nature, is sharing with GLP-1 part of its structural sequence, and also several glucoregulatory effects in mammals in an even more efficient manner. We have explored the effect of continuous administration (3 days by osmotic pump) of Ex-4 or saline (control) on bone turnover factors and bone structure in T2D and IR rats, compared to N, and the possible interaction of Ex-4 with the Wnt signalling pathway. Blood was taken before and after treatment for plasma measurements; tibiae and femurs were collected for gene expression of bone markers (RT-PCR) and structure (microCT) analysis; we also measured the mRNA levels of LRP5 - an activator of the Wnt pathway - and those of DKK1 and sclerostin (SOST) - both blockers of LRP5 activity. Compared to N-control, plasma glucose and insulin were respectively higher and lower in T2D; osteocalcin (OC) and tartrate-resistant alkaline phosphatase 5b (TRAP5b) were lower; after Ex-4, these turnover markers were further reduced in T2D and IR, while TRAP5b increased in N. Bone OC, osteoprogeterin (OPG) and receptor activator of NF-kB ligand (RANKL) mRNA were lower in T2D and IR; Ex-4 increased OC in all groups and OPG in N and IR, reduced RANKL in N and T2D but increased it in IR; the LRP5/DKK1 and LRP5/SOST mRNA ratios were similarly decreased in T2D, but in IR, the latter ratio was reduced while the former was increased; after Ex-4, both ratios augmented in N, and that of LRP5/DKK1 tended to normalize in T2D and IR. In conclusion, Ex-4 exerts osteogenic effects in T2D and IR models, and interacts with the Wnt pathway to promote bone formation.