The relationship of serum osteocalcin concentration to insulin secretion, sensitivity, and disposal with hypocaloric diet and resistance training

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.

Associations of total osteocalcin with all-cause and cardiovascular mortality in older men. The Health In Men Study

SUMMARY

In older men, both lower and higher total osteocalcin levels predict increased all-cause mortality, with comparable associations for cardiovascular and non-cardiovascular deaths. Differences in osteocalcin levels might influence glucose metabolism and thereby cardiovascular risk, or reflect changes in bone turnover thus representing a marker for poorer health outcomes.

INTRODUCTION

Reduced levels of total osteocalcin (TOC) are associated with adiposity, insulin resistance and type 2 diabetes, implying this bone-derived peptide might modulate cardiovascular risk. However, there are few longitudinal data relating TOC levels to survival. We examined associations of TOC level with all-cause and cardiovascular mortality in older men.

METHODS

We conducted a prospective cohort study of community-dwelling men aged 70-89 years. Aliquots of plasma collected at baseline (2001-2004) were assayed for TOC. Incidence and causes of death to 31 December 2008 were ascertained using data linkage. Cox regression analyses were performed with adjustment for conventional cardiovascular risk factors.

RESULTS

From 3,542 men followed for median 5.2 years there were 572 deaths (16.1%). Mortality was lowest in men with TOC levels in the second quintile (12.6%). In multivariate analyses, men with TOC in the lowest and highest quintiles of values had increased all-cause mortality (Q1 vs Q2: hazard ratio [HR], 1.36; 95% confidence interval 1.02-1.80 and Q5 vs Q2: HR, 1.53, 95% CI 1.18-1.98). Men with low TOC levels had similar HR for cardiovascular and non-cardiovascular deaths (Q1 vs Q2: HR, 1.35 and 1.30 respectively). Higher TOC levels predicted cardiovascular disease (CVD)-related mortality (Q5 vs Q2, HR, 1.69, 95% CI 1.09-2.64).

CONCLUSIONS

TOC predicts all-cause and CVD-related mortality in community-dwelling older men. However, the relationship is U shaped with men at both ends of the distribution at increased risk. Further investigation is required to clarify whether the underlying mechanisms involve altered bone turnover or relate specifically to the biological activity of osteocalcin.

Total and undercarboxylated osteocalcin predict changes in insulin sensitivity and β cell function in elderly men at high cardiovascular risk

BACKGROUND

Osteocalcin has been related to insulin secretion in experimental models. Few prospective studies have evaluated the association between circulating osteocalcin concentrations and insulin secretion and sensitivity in humans.

OBJECTIVE

The objective was to examine cross-sectional and longitudinal associations between circulating forms of osteocalcin and insulin secretion and sensitivity in elderly men at high cardiovascular risk.

DESIGN

We examined cross-sectional and longitudinal associations between serum measurements of total osteocalcin and undercarboxylated osteocalcin (ucOC) with fasting glucose, fasting insulin, HOMA-IR, and HOMA β cell function (HOMA-BCF) in 79 elderly men. We also examined the association between 2-y changes in osteocalcin and changes in fasting glucose, insulin, HOMA-IR, and HOMA-BCF.

RESULTS

In an adjusted multivariable linear regression analysis, increases in serum osteocalcin were significantly associated with an increase in HOMA-BCF (β coefficient: 2.87; 95% CI: 0.23, 5.52; P = 0.033), and changes in ucOC were linked to a decrease in HOMA-IR (β coefficient: -0.31; 95% CI: -0.60, 0.03; P = 0.032). Moreover, in subjects not taking oral antidiabetic drugs, baseline osteocalcin concentrations were positively associated with higher fasting insulin concentrations and HOMA-BCF even after adjustment for BMI, physical activity, intervention group, presence of type 2 diabetes mellitus, and baseline values of each dependent variable.

CONCLUSIONS

Changes in serum osteocalcin and ucOC are associated with an improvement in insulin secretion and sensitivity, which suggests a possible role of bone in the development of type 2 diabetes. This trial is registered at clinicaltrials.gov as ISRCTN35739639.

Interactions between Serum Adipokines and Osteocalcin in Older Patients with Hip Fracture

Introduction. Experiments on genetically modified animals have discovered a complex cross-regulation between adipokines (leptin, adiponectin) and osteocalcin. The relationships between these molecules in human osteoporosis are still unclear. We evaluated the hypothesis of a bidirectional link between adipokines and osteocalcin. Materials and Methods. In a cross-sectional study of 294 older patients with osteoporotic hip fracture, we estimated serum concentrations of leptin, adiponectin, resistin, osteocalcin, parameters of mineral metabolism, and renal function. Results. After adjustment for multiple potential confounders, serum osteocalcin concentration was inversely associated with resistin and positively with leptin, leptin/resistin ratio, and adiponectin/resistin ratio. In multivariate regression models, osteocalcin was an independent predictor of serum leptin, resistin, leptin/resistin, and adiponectin/resistin ratios. Conclusions. Our data support the bidirectional regulation between osteocalcin and adipokines, but contrary to the genetically modified animal models, in older subjects with osteoporotic hip fracture, serum osteocalcin is positively associated with leptin and inversely with resistin.

Weight-loss-associated changes in bone mineral density and bone turnover after partial weight regain with or without aerobic exercise in obese women

Background/Objectives

Moderate, long-term weight loss results in loss of bone mass in overweight or obese premenopausal women. However, whether these changes persist during weight maintenance or regain remains to be determined.

Subjects/Methods

Overweight or obese (BMI: 25.8–42.5 kg/m²) women (n=40) with at least two risk factors for the metabolic syndrome participated in this 12-mo study that examined the effects of prescribed weight loss and regain, with or without exercise, on bone turnover and on bone mineral density (BMD) in a subset of participants (n=24). During the first 6 mo, participants lost ~10% of their initial body weight via energy restriction and supervised aerobic exercise. Following weight loss, participants were randomly assigned to either an exercise or a no-exercise treatment for the regain (+50% of weight lost) phase. A one-way (time) repeated measures ANOVA tested the effects of weight loss on BMD and bone turnover, and a two-way RM ANOVA (time, exercise) was used to examine the effects of exercise during weight regain.

Results

Hip (p=0.007) and lumbar spine (p=0.05) BMD decreased with weight loss, and remained reduced after weight regain with or without exercise. Likewise, the weight-loss-associated increases in osteocalcin (p<0.001) and C-terminal peptide of type I collagen (p<0.001) persisted following weight regain, independent of exercise.

Conclusions

the results of the present study, which is the first to examine changes in bone mass and turnover during carefully controlled weight regain, suggest that weight-loss-induced perturbations in bone mass and turnover persist after partial weight regain, regardless of whether regular, weight-bearing aerobic exercise was continued.

Association of vitamin K status with adiponectin and body composition in healthy subjects: uncarboxylated osteocalcin is not associated with fat mass and body weight

Osteocalcin (OC) is a vitamin K-dependent protein found in bone and in circulation. High serum γ-carboxylated OC reflects a high, and high uncarboxylated OC (ucOC) reflects a low vitamin K status. A revolutionary hypothesis is that ucOC acts as a hormone improving glucose handling and reducing fat mass. The objective was to test the logical extrapolation of the ucOC hormone hypothesis to humans that elevated ucOC is associated with higher body weight, BMI and fat mass. In a cross-sectional analysis, the associations of vitamin K status with circulating adiponectin and body composition were investigated in 244 postmenopausal women (study I). The effects of vitamin K treatment on adiponectin, body weight and BMI were investigated in archived samples from forty-two young men and women who received varying doses of menaquinone-7 during 12 weeks (study II) and from a cohort of 164 postmenopausal women who participated in a 3-year placebo-controlled trial on 45 mg menaquinone-4 (MK-4) (study III). No association was found between vitamin K status and circulating adiponectin before or after vitamin K supplementation. A higher carboxylation of OC was significantly correlated with lower body weight, BMI and fat mass of the trunk. Women taking MK-4 maintained their baseline body weight and BMI, whereas women taking placebo showed significant increases in both indices. These findings demonstrate that a high vitamin K status of bone has no effect on circulating adiponectin in healthy people and long-term vitamin K supplementation does not increase weight in healthy postmenopausal women.

Osteocalcin as a marker of metabolic risk in healthy postmenopausal women

OBJECTIVE

Several studies have reported the role of osteocalcin on glucose and fat metabolism. In this study, we analyzed the relationship between the concentration of osteocalcin and metabolic risk factors in healthy postmenopausal women.

METHODS

Cross-sectional analyses of 54 postmenopausal women aged 56 ± 3.5 years were conducted. We recorded clinical and biochemical data of metabolic risk including fasting plasma glucose (FPG) level and evaluated the relationship between serum osteocalcin and bone formation markers.

RESULTS

Serum osteocalcin concentration was negatively correlated with FPG (β = -0.328, P = 0.035). When osteocalcin levels were divided into tertiles, we found significant differences in FPG between the highest and the lowest tertiles (84 ± 11 vs 98 ± 30 mg/dL, respectively; P = 0.029). We found significantly lower osteocalcin levels in women with impaired fasting glucose levels than in those with normoglycemia (10.7 ± 6.1 vs 17.1 ± 7.4 ng/mL, respectively; P = 0.006). We also found lower concentrations of osteocalcin in obese women versus nonobese women (14.4 ± 8.8 vs 17.3 ± 6.2 ng/mL; P = 0.034) and women with increased low-density lipoprotein cholesterol levels versus those with low LDL-c levels (14.1 ± 5.4 vs 18.9 ± 9.1 ng/mL; P = 0.045). A concentration of 13.5 ng/ mL or lower showed a sensitivity of 85.7% and a specificity of 63.8% to detect increased risk for diabetes (FPG ≥100 mg/dL). In contrast, serum levels of bone alkaline phosphatase did not correlate with any variable.

CONCLUSIONS

In this population, there is a consistent association between osteocalcin and markers of metabolic syndrome. We suggest potential usefulness of serum osteocalcin as a predictor for increased risk of diabetes in postmenopausal women.

Biology without walls: the novel endocrinology of bone

Classical studies of vertebrate physiology have usually been confined to a given organ or cell type. The use of mouse genetics has changed this approach and has rejuvenated the concept of a whole-body study of physiology. One physiological system that has been profoundly influenced by mouse genetics is skeletal physiology. Indeed, genetic approaches have identified several unexpected organs that affect bone physiology. These new links have begun to provide a plausible explanation for the evolutionary involvement of hormones such as leptin with bone physiology. These genetic approaches have also revealed bone as a true endocrine organ capable of regulating energy metabolism and reproduction. Collectively, the body of work discussed below illustrates a new and unconventional role for bone in mammalian physiology.

Epicardial adipose tissue: emerging physiological, pathophysiological and clinical features

Epicardial adipose tissue is an unusual visceral fat depot with anatomical and functional contiguity to the myocardium and coronary arteries. Under physiological conditions, epicardial adipose tissue displays biochemical, mechanical and thermogenic cardioprotective properties. Under pathological circumstances, epicardial fat can locally affect the heart and coronary arteries through vasocrine or paracrine secretion of proinflammatory cytokines. What influences this equilibrium remains unclear. Improved local vascularization, weight loss, and targeted pharmaceutical interventions could help to return epicardial fat to its physiological role. This review focuses on the emerging physiological and pathophysiological aspects of the epicardial fat and its numerous and innovative clinical applications. Particular emphasis is placed on the paracrine/endocrine properties of epicardial fat and its role in the development and progression of atherosclerosis.

The association between serum osteocalcin levels and metabolic syndrome in Koreans

The association between serum osteocalcin levels and metabolic syndrome (MS) in Korean individuals was investigated. Serum osteocalcin levels are significantly lower in subjects with MS than in those without the disease, regardless of glucose metabolism.

INTRODUCTION

Osteocalcin was recently shown to affect energy metabolism. In the present study, we investigated the possible association between serum osteocalcin concentrations and MS.

METHODS

A cross-sectional community-based survey was conducted. Serum osteocalcin, type 1 collagen C-telopeptide (CTX) and total alkaline phosphatase (ALP) concentrations were determined in 567 subjects. MS was defined according to NCEP-ATP III criteria.

RESULTS

Serum osteocalcin concentrations were significantly lower in subjects with MS than those without MS in postmenopausal women (18.923 ± 7.685 vs 22.513 ± 7.344 ng/ml, P<0.001) and marginally lower in subjects with MS than those without MS in men (14.550 ± 5.090 vs 16.125 ± 4.749 ng/ml, P=0.086) after adjustment for age and BMI. Further controlling with CTX or ALP did not affect this association in postmenopausal women; however, controlling with osteocalcin abolished the association between CTX and MS. Significant differences in serum osteocalcin levels by MS status were noted in subjects with normal glucose tolerance as well as those with abnormal glucose tolerance (P=0.032 and P<0.001, respectively). Compared with subjects with the highest quartile of osteocalcin, those in the lower quartile groups (Q1-Q3) had significantly increased risks of MS (ORs=5.18, CIs=1.15-23.42) in men. In postmenopausal women, the ORs for MS were significantly higher in the lowest quartile than in the highest quartile (ORs=5.25, CIs=2.42-11.36).

CONCLUSIONS

These findings suggest that osteocalcin is associated with MS, independently of glucose metabolism.

Relationship between serum levels of osteocalcin and atherosclerotic disease in type 2 diabetes

AIMS

To analyze the relationship between serum levels of osteocalcin and parameters of atherosclerosis in patients with type 2 diabetes mellitus (T2DM).

METHODS

This cross-sectional study of 78 patients with T2DM evaluated intima-media thickness, and the prevalence of coronary heart disease, atherosclerotic plaques and aortic calcifications. Serum osteocalcin levels were also determined by radioimmunoassay.

RESULTS

The patients' mean age was 57.8±6.4 years (duration of diabetes: 13.4 years; mean HbA(1c) level: 8.01%), and 37.2% had coronary heart disease, 56% had an abnormal intima-media thickness, 26.9% had carotid plaques and 32.1% had aortic calcifications. Coronary heart disease was associated with higher levels of osteocalcin in male vs female patients (1.95±1.36 vs 0.93±0.86 ng/mL, respectively; P=0.006). Also, higher concentrations of osteocalcin were found in female patients with vs without abnormal intima-media thicknesses (2.17±1.84 vs 1.25±0.67 ng/mL, respectively; P=0.042), carotid plaques (2.86±2.10 vs 1.43±1.09 ng/mL, respectively; P=0.03) and aortic calcifications (2.85±1.97 vs 1.26±0.83 ng/mL, respectively; P=0.002). Serum osteocalcin levels were associated with coronary heart disease on multivariate logistic regression (odds ratio: 2.27, 95% confidence interval: 1.21-4.25; P=0.01).

CONCLUSION

In T2DM patients, serum osteocalcin levels were associated with parameters of atherosclerosis, suggesting that osteocalcin is involved not only in bone metabolism, but also in atherosclerotic disease.

An old friend in a new light: the role of osteocalcin in energy metabolism

Accumulating evidence suggests interactions between bone and energy metabolism, which may affect the risk of cardiovascular disease. Recent animal studies indicate that osteocalcin (OC) plays a key role in the coordinated regulation of glucose and insulin metabolism while insulin receptors on osteoblasts may regulate bone turnover and circulating OC levels. Association studies, weight loss interventions, and observational data lend some support to the existence and relevance of these mechanisms in humans. However, corroborating evidence from pharmacologic interventions in either bone or glucose metabolism is limited by the number, design, and complex pharmacological effects of the drugs used. Furthermore, such clinical trials are complicated by the alteration of metabolic feedback mechanisms in the insulin resistant state. Purpose-designed studies are needed to further establish the existence and significance of the role of OC and its subfractions in human insulin metabolism. In this review we summarize existing animal evidence regarding the role of OC and its subfractions in bone and energy metabolism and assess current clinical trial evidence relating to the significance and consequences of this relationship in humans.

Different effects on bone strength and cell differentiation in pre pubertal caloric restriction versus hypothalamic suppression

Hypothalamic amenorrhea and energy restriction during puberty affect peak bone mass accrual. One hypothesis suggests energy restriction alters hypothalamic function resulting in suppressed estradiol levels leading to bone loss. However, both positive and negative results have been reported regarding energy restriction and bone strength. Therefore, the purpose of this study was to investigate energy restriction and hypothalamic suppression during pubertal onset on bone mechanical strength and the osteogenic capacity of bone marrow-derived cells in two models: female rats treated with gonadotropin releasing hormone antagonists (GnRH-a) or 30% energy restriction. At 23 days of age, female Sprague Dawley rats were assigned to three groups: control group (C, n=10), GnRH-a group (n=10), and Energy Restriction (ER, n=12) group. GnRH-a animals received daily injections for 27 days. The animals in the ER group received 70% of the control animals' intake. After sacrifice (50 days of age), body weight, uterine and muscle weights were measured. Bone marrow-derived stromal cells were cultured and assayed for proliferation and differentiation into osteoblasts. Outcome measures included bone strength, bone histomorphometry and architecture, serum IGF-1 and osteocalcin. GnRH-a suppressed uterine weight, decreased osteoblast proliferation, bone strength, trabecular bone volume and architecture compared to control. Elevated serum IGF-1 and osteocalcin levels and body weight were found. The ER model had an increase in osteoblast proliferation compared to the GnRH-a group, similar bone strength relative to body weight and increased trabecular bone volume in the lumbar spine compared to control. The ER animals were smaller but had developed bone strength sufficient for their size. In contrast, suppressed estradiol via hypothalamic suppression resulted in bone strength deficits and trabecular bone volume loss. In summary, our results support the hypothesis that during periods of nutritional stress the increased vertebral bone volume may be an adaptive mechanism to store mineral which differs from suppressed estradiol resulting from hypothalamic suppression.

Serum levels of osteocalcin are inversely associated with the metabolic syndrome and the severity of coronary artery disease in Chinese men

OBJECTIVE

Osteocalcin is a bone-derived protein and has been shown to play an important role in regulating glucose and fat metabolism. We therefore investigated the association of serum levels of osteocalcin with the metabolic syndrome (MS) and coronary atherosclerosis in Chinese men.

RESEARCH DESIGN AND METHODS

Serum osteocalcin levels were measured by an electrochemiluminescence immunoassay in 181 men who underwent coronary angiography, and their association with the MS and the severity of coronary artery disease (CAD) were studied.

RESULTS

Osteocalcin levels in patients with the MS were significantly lower compared with those in non-MS subjects (P < 0·001) and decreased correspondingly with the increasing number of components of the MS (P < 0·001). Multiple logistic regression analysis demonstrated that osteocalcin was independently associated with the MS (OR = 0·060, 95%CI: 0·005-0·651). In multiple stepwise regression analysis, waist circumference (P = 0·001) and fasting plasma glucose (P = 0·002) were independently associated with serum osteocalcin. Subgroup analysis in 60 subjects with normal glucose tolerance showed that serum osteocalcin decreased significantly in patients with CAD compared with those without CAD (P = 0·029) and decreased significantly as the number of stenotic vessels increased (P = 0·033). Furthermore, serum osteocalcin showed an independent correlation with coronary atherosclerosis index (standardized β = -0·497, P = 0·003).

CONCLUSION

Serum osteocalcin is inversely associated with the MS as well as the severity of coronary atherosclerosis in Chinese men, supporting the new concept that bone has the reciprocal regulation with energy metabolism.

Osteocalcin-insulin relationship in obese children: a role for the skeleton in energy metabolism

OBJECTIVE

Osteocalcin is a bone-specific protein secreted by osteoblasts and often used as a bone formation biomarker. Rodent studies have reported a hormonal role of osteocalcin on glucose metabolism, increasing insulin secretion and sensitivity and increasing energy expenditure. However, it is unknown whether osteocalcin fulfils the same function in humans.

METHODS

We investigated the relationship between serum osteocalcin and insulin concentrations in 27 prepubertal obese children (9-12 years old) randomly divided into two groups, one of which entered a physical training programme, and 16 nonobese control children. Whole body bone mineral density (WB-BMD), serum osteocalcin, circulating insulin and adiponectin were measured at baseline and after 6 months.

RESULTS

Trained and untrained obese children had higher WB-BMD than controls at baseline. Trained children also displayed a significant insulin increase and a significant adiponectin decrease while osteocalcin was increased compared to untrained obese children. Significant linear correlations between WB-BMD and adiponectin, delta BMD (variation between baseline and after-training values) and delta adiponectin, insulin and osteocalcin, delta insulin and delta osteocalcin, delta insulin and delta under-carboxylated osteocalcin were found only in trained obese children with no significant relationship in control and untrained obese children.

CONCLUSIONS

In trained obese children, correlations indicate that when BMD is increased, osteocalcin is increased and insulin lowered. This suggests that increased BMD is associated with increased energy metabolism and a decreased level of insulin. We thus report statistically significant relationships between the skeleton (osteocalcin) and energy metabolism (insulin), suggesting a regulatory hormonal loop including osteocalcin and insulin.

Serum concentrations of osteocalcin, procollagen type 1 N-terminal propeptide and beta-CrossLaps in obese subjects with varying degrees of glucose tolerance

AIMS

To evaluate serum levels of osteocalcin (OC), procollagen type 1 N-terminal propeptide (P1PN) and beta-CrossLaps (beta-CTx) in obese subjects and their relationship with glucose metabolism parameters.

SUBJECTS

Sixty-four obese patients classified according to their glucose tolerance.

DESIGN

Case-control study.

MEASUREMENTS

A 75-g oral glucose tolerance test was performed with determinations of glucose and insulin between 0 and 120 min. Serum concentrations of OC, P1PN and beta-CTx were quantified in baseline samples.

RESULTS

Patients with type 2 diabetes (T2D, n = 24) exhibited OC serum levels (2·6 ± 1·0 nm) significantly lower than those found in subjects with normal glucose tolerance (NGT, n = 20, 3·9 ± 1·5 nm, P < 0·01). We found no significant differences in P1NP and beta-CTX levels among patients with NGT, prediabetes and T2D. Multiple regression analysis showed that serum OC concentration, but not P1NP or beta-CTx levels, was independently related to 2-h plasma glucose.

CONCLUSION

Obese patients with T2D showed significantly reduced levels of OC in comparison with patients with lower degrees of glucose tolerance derangement. Our results also suggest that OC was the only bone marker independently related to the degree of glucose metabolism derangement in these patients.

Low serum osteocalcin level is a potential marker for metabolic syndrome: results from a Chinese male population survey

Osteocalcin has been recognized as a bone-derived hormone to regulate energy metabolism recently. Little is known about the role of osteocalcin as regards metabolic syndrome (MetS) in a Chinese population. Components of MetS, osteocalcin, body mass index (BMI), and prevalence of MetS were assessed in 2344 men aged 20 to 69 years who participated in the population-based Fangchenggang Area Male Health and Examination Survey, which was carried out in Guangxi province of China from September 2009 to December 2009. Osteocalcin had a statistically significant positive correlation with high-density lipoprotein cholesterol and a negative relationship with blood pressure, glucose, triglycerides, waist circumference, and BMI after adjustment for age (all P < .001). The strongest correlation was observed between osteocalcin and BMI (r = -0.26). In a multivariate analysis, decreased odds ratios (ORs) for the MetS and its components as well were observed from the first to the fourth osteocalcin quartiles. After adjustment for BMI, the OR decreased substantially. Statistically significant difference still existed in MetS (OR, 1.77; 95% confidence interval [CI], 1.10-2.85), hypertriglyceridemia (OR, 1.66; 95% CI, 1.22-2.27), hyperglycemia (OR, 1.42; 95% CI, 1.05-1.92), and low high-density lipoprotein cholesterol (OR, 1.83; 95% CI, 1.03-3.24) when these risks were compared in the lowest quartile of osteocalcin levels with those in the highest quartile. In a Chinese male population, we firstly identified an inverse association of serum osteocalcin levels with MetS, independent from the well-known MetS risk factors. This may represent a further mechanism for the elevated cardiovascular disease or type 2 diabetes mellitus risk.

Lower uncarboxylated osteocalcin concentrations in children with prediabetes is associated with beta-cell function

CONTEXT

Although animal studies suggest that it is the uncarboxylated rather than carboxylated form of osteocalcin that affects glucose homeostasis, the human data are scant and equivocal.

OBJECTIVE

This study investigated associations of uncarboxylated and carboxylated forms of osteocalcin with insulin sensitivity and β-cell function in 140 overweight prepubertal children (43% female, 46% black, 84% obese) with normal glucose levels (n = 99) and prediabetes (n = 41).

METHODS

An oral glucose tolerance test was used to identify prediabetes and for measurement of insulin sensitivity (Matsuda index), β-cell function [oral glucose tolerance test derived insulinogenic index and disposition index (DI(OGTT))] and uncarboxylated and carboxylated forms of osteocalcin. Visceral adipose tissue (VAT) was assessed using magnetic resonance imaging.

RESULTS

After controlling for age, sex and race, lower uncarboxylated osteocalcin concentrations, Matsuda index, insulinogenic index, and DI(OGTT) and higher VAT levels were found in the prediabetes vs. normal-glucose group (all P < 0.03). Carboxylated osteocalcin levels were not different between groups. Multiple linear regression adjusting for age, sex, race, and VAT revealed that uncarboxylated osteocalcin was associated with insulinogenic index and DI(OGTT) (β = 0.34, 0.36, respectively, both P < 0.04) in the prediabetes group but not the normal-glucose group. In both the normal-glucose and prediabetes groups, carboxylated osteocalcin was associated with insulin sensitivity (β = 0.26, 0.47, respectively, both P < 0.02).

CONCLUSIONS

These data suggest that the lower uncarboxylated osteocalcin concentrations found in children with prediabetes may be associated with β-cell dysfunction. In addition, our findings between carboxylated osteocalcin and insulin sensitivity suggest that carboxylated osteocalcin plays a role in human glucose homeostasis.

Osteocalcin: a new link between bone and energy metabolism. Some evolutionary clues

PURPOSE OF REVIEW

Recent findings suggest that the bone is an active regulator of energy and glucose metabolism. The purpose of this review is to summarize current evidence in humans.

RECENT FINDINGS

Both cross-sectional and longitudinal studies support osteocalcin as an active regulator of carbohydrate metabolism in humans, being the muscular load of physical activity one of the possible links between the osteoblast and the insulin axis. This axis could also have been involved in the modulation of nonalcoholic steatohepatitis. The osteoblast-to-insulin axis seems to act paradoxically in patients with increased growth hormone (acromegaly) and during bone repair. Some possible evolutionary implications are suggested.

SUMMARY

Osteocalcin may have a role in the regulation of systemic energy metabolism, given the common origin of the osteoblast with the two other cells implicated (adipocytes and muscle cells). Bioactivity of circulating human carboxylated and uncarboxylated osteocalcin should be characterized in depth, especially in those patients with increased concentrations (renal failure). Osteocalcin is one of the clues in the interaction between calcium and glucose metabolism, and the discovery of the osteocalcin receptor will aid in the study of these relationships.

Osteocalcin and atherosclerosis: a complex relationship

The relationship between OC and atherosclerosis in different clinical situations confirms the interplay that exists among bone cells and vascular system. The recently demonstrated effects of OC on glucose metabolism and visceral fat in healthy subjects and T2DM patients, contribute to the knowledge of the role of OC in the pathogenesis of cardiovascular disease.

Relationship between bone biochemical markers versus glucose/lipid metabolism and atherosclerosis; a longitudinal study in type 2 diabetes mellitus

AIM

Although accumulating evidence suggests that osteocalcin, which is secreted in circulation specifically from osteoblasts, is involved in glucose and lipid metabolism, it is unclear whether serum osteocalcin is associated with atherosclerosis parameter in humans.

SUBJECTS AND METHODS

We monitored chronological changes in bone turnover markers and a parameter of atherosclerosis, plaque score (PS), during glycemic control in Japanese patients with type 2 diabetes, and analyzed relationships among these parameters.

RESULTS

Multiple regression analysis showed that changes in osteocalcin were negatively correlated with changes in HbA(1c) (β = -0.38, p = 0.01). Baseline osteocalcin was negatively correlated with changes in triglycerides (β = -0.29, p = 0.03) and positively with changes in HDL-cholesterol (β = 0.31, p = 0.03), and that changes in osteocalcin were negatively correlated with baseline triglyceride (β = -0.35, p = 0.02). Changes in osteocalcin were positively correlated with baseline PS (β = 0.35, p = 0.01) and negatively with changes in PS (β = -0.30, p < 0.05) independent of other conventional risk factors of atherosclerosis.

CONCLUSION

These findings indicated the association between serum osteocalcin and glucose and lipid metabolism as well as an atherosclerosis parameter independently of other atherosclerosis-related risk factors in patients with type 2 diabetes, suggesting that osteocalcin is important for not only bone metabolism but also glucose and lipid metabolism as well as atherosclerosis.

The role of osteocalcin in the endocrine cross-talk between bone remodelling and energy metabolism

Bone remodelling, which maintains bone mass constant during adulthood, is an energy-demanding process. This, together with the observation that the adipocyte-derived hormone leptin is a major inhibitor of bone remodelling, led to the hypothesis that bone cells regulate energy metabolism through an endocrine mechanism. Studies to test this hypothesis identified osteocalcin, a hormone secreted by osteoblasts, as a positive regulator of insulin secretion, insulin resistance and energy expenditure. Remarkably, insulin signalling in osteoblasts is a positive regulator of osteocalcin production and activation via its ability to indirectly enhance bone resorption by osteoclasts. In contrast, leptin is a potent inhibitor of osteocalcin function through its effect on the sympathetic tone. Hence, osteocalcin is part of a complex signalling network between bone and the organs more classically associated with the regulation of energy homeostasis, such as the pancreas and adipose tissue. This review summarises the molecular and cellular bases of the present knowledge on osteocalcin biology and discusses the potential relevance of osteocalcin to human metabolism and pathology.

Cross-sectional evidence of a signaling pathway from bone homeostasis to glucose metabolism

CONTEXT

Preclinical studies suggested the existence of a signaling pathway connecting bone and glucose metabolisms. Supposedly leptin modulates osteocalcin bioactivity, which in turn stimulates insulin and adiponectin secretion, and β-cell proliferation.

OBJECTIVE

The objective of the investigation was to study the reciprocal relationships of adiponectin, leptin, osteocalcin, insulin resistance, and insulin secretion to verify whether such relationships are consistent with a signaling pathway connecting bone homeostasis and glucose metabolism.

DESIGN

This was a cross-sectional analysis.

SETTING

The study was conducted with community-dwelling volunteers participating in the Baltimore Longitudinal Study of Aging.

PARTICIPANTS

Two hundred eighty women and 300 men with complete data on fasting plasma adiponectin, leptin, and osteocalcin, oral glucose tolerance test (plasma glucose and insulin values available at t = 0, 20, and 120 min), and anthropometric measures participated in the study.

MAIN OUTCOME MEASURES

Linear regression models were used to test independent associations of adiponectin, osteocalcin, and leptin with the indices of insulin resistance and secretion. The expected reciprocal relationship between different biomarkers was verified by structural equation modeling.

RESULTS

In linear regression models, leptin was strongly associated with indices of both insulin resistance and secretion. Both adiponectin and osteocalcin were negatively associated with insulin resistance. Structural equation modeling revealed a direct inverse association of leptin with osteocalcin; a direct positive association of osteocalcin with adiponectin; and an inverse relationship of osteocalcin with insulin resistance and adiponectin with insulin resistance and secretion, which is cumulatively consistent with the hypothesized model.

CONCLUSIONS

Bone and glucose metabolisms are probably connected through a complex pathway that involves leptin, osteocalcin, and adiponectin. The clinical relevance of such a pathway for bone pathology in diabetes should be further investigated.

The effect of acute exercise on undercarboxylated osteocalcin in obese men

SUMMARY

The purpose of this study was to examine if the reduction in glucose post-exercise is mediated by undercarboxylated osteocalcin (unOC). Obese men were randomly assigned to do aerobic or power exercises. The change in unOC levels was correlated with the change in glucose levels post-exercise. The reduction in glucose post-acute exercise may be partly related to increased unOC.

INTRODUCTION

Osteocalcin (OC) in its undercarboxylated (unOC) form may contribute to the regulation of glucose homeostasis. As exercise reduces serum glucose and improves insulin sensitivity in obese individuals and individuals with type 2 diabetes (T2DM), we hypothesised that this benefit was partly mediated by unOC.

METHODS

Twenty-eight middle-aged (52.4 ± 1.2 years, mean ± SEM), obese (BMI = 32.1 ± 0.9 kg m(-2)) men were randomly assigned to do either 45 min of aerobic (cycling at 75% of VO(2peak)) or power (leg press at 75% of one repetition maximum plus jumping sequence) exercises. Blood samples were taken at baseline and up to 2 h post-exercise.

RESULTS

At baseline, unOC was negatively correlated with glucose levels (r = -0.53, p = 0.003) and glycosylated haemoglobin (HbA1c) (r = -0.37, p = 0.035). Both aerobic and power exercises reduced serum glucose (from 7.4 ± 1.2 to 5.1 ± 0.5 mmol L(-1), p = 0.01 and 8.5 ± 1.2 to 6.0 ± 0.6 mmol L(-1), p = 0.01, respectively). Aerobic exercise significantly increased OC, unOC and high-molecular-weight adiponectin, while power exercise had a limited effect on OC and unOC. Overall, those with higher baseline glucose and HbA1c had greater reductions in glucose levels after exercise (r = -0.46, p = 0.013 and r = -0.43, p = 0.019, respectively). In a sub-group of obese people with T2DM, the percentage change in unOC levels was correlated with the percentage change in glucose levels post-exercise (r = -0.51, p = 0.038).

CONCLUSIONS

This study reports that the reduction in serum glucose post-acute exercise (especially aerobic exercise) may be partly related to increased unOC.

Circulating osteocalcin is increased in early-stage diabetes

We aimed to examine whether circulating levels of osteocalcin, bone formation marker secreted from osteoblast, are changed in glucose-intolerant subjects without taking glucose lowering agent, because bone metabolism is reportedly related to glucose metabolism in animal and human studies. According to 75 g oral glucose tolerance test (75 g-OGTT), all subjects (47.6 ± 10.2 years of age; 45 men and 10 women) were divided into three categories: normal glucose tolerance (NGT, n = 39), prediabetes (PDM, n = 11) that included impaired fasting glucose (IFG) and impaired glucose tolerance (IGT), and diabetes (T2DM, n = 5). Serum osteocalcin levels were increased in T2DM as compared to NGT. In all the participants, simple regression analysis model revealed positive correlation of osteocalcin with plasma glucose at 120 min, G(120), on 75 g-OGTT, negative with both creatinine and Ln(CRP), but not significantly with fasting plasma glucose. Osteocalcin and leptin were independent variables for G(120) (P = 0.026 and 0.035, respectively). In multinomial logistic analysis leptin (PDM vs. NGT: P = 0.02 Odds ratio (OR) of 1.05, 95% confidence intervals, 1.007-1.084) and osteocalcin (T2DM vs. NGT: P = 0.038, OR 10.8, 1.13-102.4) were independently associated. We conclude that circulating osteocalcin and leptin are related to glucose intolerant state.

The osteoblast: an insulin target cell controlling glucose homeostasis

The past five years have witnessed the emergence and discovery of unexpected functions played by the skeleton in whole-organism physiology. Among these newly described tasks is the role of bone in the control of energy metabolism, which is achieved through the secretion of osteocalcin, an osteoblasts-derived hormone regulating insulin secretion, insulin sensitivity, and energy expenditure. These initial findings raised several fundamental questions on the nature of insulin action in bone. Discoveries made independently by our two groups have provided answers recently to some of these questions. Through the analysis of mice lacking insulin receptor (InsR) only in osteoblasts, we found that insulin signaling in these cells favors whole-body glucose homeostasis. Importantly, this function of insulin signaling in osteoblasts was achieved through the negative regulation of osteocalcin carboxylation and bioavailability. Our studies also established that insulin signaling in osteoblasts was a positive regulator not only of postnatal bone acquisition but also of bone resorption. Interestingly, it appears that insulin signaling in osteoblasts induced osteocalcin activation by stimulating osteoclast activity. Indeed, the low pH generated during bone resorption is a sufficient means to decarboxylate osteocalcin. Our findings establish that the osteoblast is an important target used by insulin to control whole-body glucose homeostasis and identify bone resorption as the mechanism regulating osteocalcin activation.

Serum osteocalcin level is positively associated with insulin sensitivity and secretion in patients with type 2 diabetes

OBJECTIVE

Bone is being recognized as an endocrine organ. Although previous animal studies showed that osteocalcin stimulated the expression of insulin in islets and of adiponectin in adipocytes with increased insulin secretion and sensitivity, the associations of serum osteocalcin with those parameters remain unclear in humans.

METHODS

In this cross-sectional study, we employed 101 postmenopausal women and 152 men with type 2 diabetes, who have not taken drugs for diabetes or osteoporosis. We also examined 75 g oral glucose tolerance test (OGTT) in 18 postmenopausal women and 20 men who visited our clinic for medical check-up for diabetes.

RESULTS

In both postmenopausal women and men, multiple regression analysis adjusted for age, body mass index, and serum creatinine showed that serum osteocalcin level was significantly and negatively associated with fasting plasma glucose, HbA(1c), %Trunk fat, and homeostasis model assessment (HOMA) for insulin resistance (p<0.05), and positively with HOMA for beta-cell function (p<0.05). In addition, significant positive association of serum osteocalcin level with serum adiponectin level was found in postmenopausal women (p<0.05), but not in men. In the OGTT examinations, subjects were divided into tertiles by their serum osteocalcin levels in each gender. Postmenopausal women in the lowest tertile showed hyperglycemia and hyperinsulinemia compared to those in the highest tertile after oral glucose loading (p<0.05). Men in the lowest tertile also exhibited hyperinsulinemia (p<0.05), while hyperglycemia was not found.

CONCLUSION

These findings indicate that serum osteocalcin level is positively associated with insulin sensitivity and secretion in Japanese patients with type 2 diabetes.

Prominent bone loss mediated by RANKL and IL-17 produced by CD4+ T cells in TallyHo/JngJ mice

Increasing evidence that decreased bone density and increased rates of bone fracture are associated with abnormal metabolic states such as hyperglycemia and insulin resistance indicates that diabetes is a risk factor for osteoporosis. In this study, we observed that TallyHo/JngJ (TH) mice, a polygenic model of type II diabetes, spontaneously developed bone deformities with osteoporotic features. Female and male TH mice significantly gained more body weight than control C57BL/6 mice upon aging. Interestingly, bone density was considerably decreased in male TH mice, which displayed hyperglycemia. The osteoblast-specific bone forming markers osteocalcin and osteoprotegerin were decreased in TH mice, whereas osteoclast-driven bone resorption markers such as IL-6 and RANKL were significantly elevated in the bone marrow and blood of TH mice. In addition, RANKL expression was prominently increased in CD4+ T cells of TH mice upon T cell receptor stimulation, which was in accordance with enhanced IL-17 production. IL-17 production in CD4+ T cells was directly promoted by treatment with leptin while IFN-γ production was not. Moreover, blockade of IFN-γ further increased RANKL expression and IL-17 production in TH-CD4+ T cells. In addition, the osteoporotic phenotype of TH mice was improved by treatment with alendronate. These results strongly indicate that increased leptin in TH mice may act in conjunction with IL-6 to preferentially stimulate IL-17 production in CD4+ T cells and induce RANKL-mediated osteoclastogenesis. Accordingly, we propose that TH mice could constitute a beneficial model for osteoporosis.

Interplay between serum osteocalcin and insulin sensitivity in primary hyperparathyroidism

Osteocalcin (OC) has been proposed as a regulator of insulin sensitivity in both humans and other animals. Primary hyperparathyroidism (PHPT) is characterized by high OC levels and insulin resistance. The aim of this study was to evaluate whether in PHPT the link between OC levels and blood markers of insulin resistance was maintained. In a consecutive series of 219 adult PHPT patients, serum OC as well as fasting insulin and glucose levels were measured. Insulin sensitivity was estimated by homeostatic model assessment (HOMA2-S%). The same parameters were evaluated in a subgroup of 45 patients after parathyroidectomy (PTX). PHPT patients were characterized by markedly high OC levels. After subdividing them according to glucose tolerance, it was found that OC was similar in subjects with normal glucose tolerance (NGT) and impaired glucose tolerance (IGT), while diabetic subjects had lower serum OC than those with NGT (P < 0.02) or IGT (P < 0.04). OC was negatively associated with fasting glucose and positively associated with HOMA2-S%. OC independently predicted HOMA2-S% in a multivariate analysis. In the subgroup of surgically cured PHPT patients, OC levels significantly decreased after PTX, while HOMA2-S% did not change. Our findings indicate that in PHPT there is a positive relationship between OC and glucose metabolism, OC being one of the predictors of insulin sensitivity. However, data in surgically cured patients, showing OC normalization in spite of unchanged HOMA2-S%, suggest that OC does not likely play a major role in affecting insulin sensitivity in PHPT.

Osteocalcin: an endocrine link between bone and glucose metabolism

Impaired glucose metabolism is common and contributes to the risk of diabetes and cardiovascular disease. Deletion of the gene for the osteoblast-derived protein, osteocalcin, leads to insulin resistance in mice, while the addition of osteocalcin increases insulin secretion from β-cells and adiponectin expression in adipocytes. Osteocalcin deficiency in γ-carboxyl groups, undercarboxylated osteocalcin, was found to improve insulin secretion and sensitivity in experiments. Recent studies have examined the relevance of these findings to glucose metabolism and cardiovascular risk in humans. Low total osteocalcin levels are associated with insulin resistance, diabetes and metabolic syndrome in observational studies. New therapeutic approaches to diabetes and heart disease may be anticipated if this bone-derived protein is involved in the regulation of glucose metabolism and cardiovascular risk.

Relationship between serum osteocalcin and glycaemic variability in Type 2 diabetes

1. Recent reports have described the role of osteocalcin in glucose metabolism and glycaemic variability has been proven to be associated with an increased risk of diabetes complications. However, the relationship between osteocalcin and glycaemic variability remains unclear. The aim of the present study was to examine the relationship between serum osteocalcin and glycaemic variability, as determined by a continuous glucose monitoring (CGM) system in patients with Type 2 diabetes mellitus (T2DM). 2. Fifty-nine T2DM patients with glycosylated haemoglobin (HbA1c) levels between 7.0% and 10.9% were recruited to the present study. Biochemical information and CGM parameters were collected at baseline and after 8 weeks of antihyperglycaemic therapy (either sulphonylurea, sulphonylurea + an α-glucosidase inhibitor or insulin + metformin combination therapy). 3. Compared with baseline, serum osteocalcin increased significantly (P = 0.014), whereas parameters related to glucose variability, including the mean amplitude of glycaemic excursions (MAGE) and the standard deviation of blood glucose values, decreased significantly (P < 0.001) after the 8 week treatment period. At baseline, there was a positive correlation between serum osteocalcin levels and fasting C-peptide levels (P = 0.004) and homeostatic model assessment of β-cell function (P = 0.048), but a negative correlation between serum osteocalcin levels and fasting plasma glucose (P = 0.023), HbA1c (P = 0.020), glycated albumin (P = 0.019) and 24 h mean blood glucose (P < 0.001). Multiple stepwise regression analysis indicated that baseline osteocalcin was the single parameter that best predicted the change in MAGE (β = -0.122; P = 0.039). 4. In conclusion, serum osteocalcin concentrations increased with improved glucose control. High initial osteocalcin levels were associated with subsequent improvements in glucose variability during glucose-lowering treatment.

Serum undercarboxylated osteocalcin was inversely associated with plasma glucose level and fat mass in type 2 diabetes mellitus

Although recent animal studies have shown that undercarboxylated osteocalcin acts as a hormone regulating glucose metabolism and fat mass, little is known about the relationships in humans. We reported here for the first time that undercarboxylated osteocalcin were associated with glucose/fat metabolism in patients with type 2 diabetes.

INTRODUCTION

Recent studies have shown that undercarboxylated osteocalcin (ucOC) acts as a hormone regulating glucose metabolism and fat mass. We investigated the relationship between ucOC as well as other bone turnover markers [serum OC, bone-specific alkaline phosphatase (BAP), and urinary N-terminal cross-linked telopeptide of type-I collagen] versus serum levels of glucose, fasting serum C-peptide, and adiponectin as well as the amount of fat mass in type 2 diabetes.

METHODS

A total of 180 men and 109 postmenopausal women were consecutively recruited, and radiographic and biochemical characteristics were collected. Fat mass was measured by dual X-ray absorptiometry (DXA) and computed tomography (CT).

RESULTS

In men, ucOC negatively correlated with percent trunk fat (%trunk fat; by DXA) and visceral/subcutaneous fat ratio (by CT) as well as fasting plasma glucose and HbA(1c) (at least p < 0.05). Multiple regression analysis showed that these associations were still significant independent of age, duration of diabetes, body stature, and renal function as well as glucose or fat metabolism, whereas BAP, another bone formation marker, did not correlate with any variable. On the other hand, although ucOC also negatively correlated with %fat and %trunk fat as well as HbA(1c) (at least p < 0.05) in postmenopausal women, we found no significant association in multiple regression analysis.

CONCLUSIONS

These findings suggest that ucOC is associated with plasma glucose level and fat mass in men with type 2 diabetes.

Bone metabolism and fracture risk in type 2 diabetes mellitus [Review]

Osteoporosis and type 2 diabetes mellitus (T2DM) are now prevalent in aging and westernized societies, and adversely affect the health of the elderly people by causing fractures and vascular complications, respectively. Recent experimental and clinical studies show that both disorders are etiologically related to each other through the actions of osteocalcin and adiponectin. Meta-analyses of multiple clinical studies show that hip fracture risk of T2DM patients is increased to 1.4 to 1.7-folds, although BMD of the patients is not diminished. Vertebral fracture risk of T2DM patients is also increased, and BMD is not useful for assessing its risk. These findings suggest that bone fragility in T2DM depends on bone quality deterioration rather than bone mass reduction. Thus, surrogate markers are needed to replace the insensitivity of BMD in assessing fracture risks of T2DM patients. Markers related to advanced glycation end products as well as insulin-like growth factor-I may be such candidates, because these substances were experimentally shown to modulate bone quality in DM. In practice, it is important for physicians to assess fracture risk in T2DM patients by evaluating prior VFs and fracture histories using spine X-ray and interview, respectively, until the usefulness of surrogate markers is established.

Circulating osteocalcin concentrations are associated with parameters of liver fat infiltration and increase in parallel to decreased liver enzymes after weight loss

SUMMARY

The expression of liver genes was associated with insulin action in osteocalcin knockout mice. Our findings suggest that osteocalcin may play a role in the development of insulin resistance-associated fatty liver disease.

INTRODUCTION

The expression of insulin target genes was decreased in the liver of mice lacking osteocalcin. We aimed to explore the association of liver enzymes with osteocalcin.

METHODS

The associations were evaluated in a cross-sectional study (266 men) and following weight loss in 28 obese subjects (nine male, 19 females).

RESULTS

In the cross-sectional study, circulating osteocalcin concentration was negatively associated with alanine transaminase (ALT) (p = 0.002) and aspartate transaminase (AST) levels (p = 0.008). These associations were especially significant in non-obese subjects (n = 191). In a multiple linear regression analysis, age (p = 0.008), insulin sensitivity (p = 0.001), and osteocalcin (p = 0.04) independently contributed to 22% of ALT variance in these latter subjects. In the weight loss study, the increase in circulating osteocalcin concentration (+70.6 ± 29.3 vs. +32 ± 13.5%, p = 0.021) was significantly greater in subjects with the highest decrease in ALT levels, despite similar baseline BMI, insulin resistance and degree of weight loss than remaining subjects. In fact, the change in ALT levels were linearly associated with those of osteocalcin (r = -0.55, p = 0.003).

CONCLUSIONS

In summary, our findings suggest a bone-liver axis in which osteocalcin might be the active regulator.

Bone fragility in type 2 diabetes mellitus

The number of patients with osteoporosis or type 2 diabetes mellitus (T2DM) is increasing in aging and westernized societies. Both disorders predispose elderly people to disabling conditions by causing fractures and vascular complications, respectively. It is well documented that bone metabolism and glucose/fat metabolism are etiologically related to each other through osteocalcin action and Wnt signaling. Bone fragility in T2DM, which is not reflected by bone mineral density (BMD), depends on bone quality deterioration rather than bone mass reduction. Thus, surrogate markers are needed to replace the insensitivity of BMD in assessing fracture risks of T2DM patients. Pentosidine, the endogenous secretory receptor for advanced glycation endproducts, and insulin-like growth factor-I seem to be such candidates, although further studies are required to clarify whether or not these markers could predict the occurrence of new fractures of T2DM patients in a prospective fashion.

Serum osteocalcin is related to abdominal obesity in Korean obese and overweight men

BACKGROUND

Growing evidence suggests a link between body composition and serum osteocalcin. Here we examined such an association between serum osteocalcin and body composition in Korean obese men.

METHODS

Eighty-six men, aged 20 to 76 years, who visited the obesity clinic at the Division of Family Medicine, Severance hospital, Seoul, Korea were recruited for this study. Abdominal fat computed tomography (CT) scans were performed to measure the visceral fat area (VFA) and subcutaneous fat area (SFA).

RESULTS

Serum osteocalcin levels were negatively correlated with age, VFA, and VFA/SFA ratio. In addition, serum osteocalcin levels were significantly decreased in obese and overweight subjects with visceral obesity (12.7±3.2) compared to those without visceral obesity (18.6±4.9). Multiple regression analysis showed that serum osteocalcin levels were associated with VFA in obese and overweight men after adjustment for age, insulin resistance, current smoking, alcohol consumption and adiposity indices.

CONCLUSION

Osteocalcin was inversely related to visceral obesity in Korean obese and overweight men. These results suggest cross-talk between bone and adipose tissue.

Baseline serum total adiponectin level is positively associated with changes in bone mineral density after 1-year treatment of type 2 diabetes mellitus

Although recent clinical studies have shown that serum adiponectin level was negatively associated with bone mineral density (BMD), serum adiponectin action on bone metabolism in humans is still unclear. We investigated the relationships between serum levels of total and high-molecular weight (HMW) adiponectin and its ratio (HMW-total ratio) vs chronological changes in BMD at the lumbar spine, femoral neck (FN), and one third of the radius after 1-year treatment of type 2 diabetes mellitus in 32 Japanese patients. Serum total adiponectin, but not HMW adiponectin or HMW-total ratio, was significantly and positively correlated with percentage change in FN-BMD (r = 0.35, P < .05). Multiple regression analysis adjusted for age, duration of diabetes, sex, body height, body weight, waist circumference, serum creatinine, and hemoglobin A(1c) showed that serum total adiponectin was still significantly and positively correlated with percentage change in FN-BMD (r = 0.65, P < .01). On the other hand, no significant relationships were found between serum levels of hemoglobin A(1c), pentosidine, bone formation markers (bone-specific alkaline phosphatase and osteocalcin), or a bone resorption marker (urinary N-terminal cross-linked telopeptide of type-I collagen) vs percentage change in BMD at any site. These findings suggest that serum total adiponectin could be clinically useful for predicting BMD change during treatment of type 2 diabetes mellitus. Adiponectin might protect against BMD reduction in patients with type 2 diabetes mellitus.

Bone and glucose metabolism: a two-way street

Evidence from rodent models indicates that undercarboxylated osteocalcin (ucOC), a product of osteoblasts, is a hormone affecting insulin production by the pancreas and insulin sensitivity in peripheral tissues, at least in part through enhanced secretion of adiponectin from adipocytes. Clinical research to test whether this relationship is found in humans is just beginning to emerge. Cross-sectional studies confirm associations between total osteocalcin (OC), ucOC and glucose metabolism but cannot distinguish causality. To date, longitudinal studies have not provided a consistent picture of the effects of ucOC or OC on fasting glucose and insulin sensitivity. Further exploration into the physiological and mechanistic effects of ucOC and OC, in rodent models and clinical studies, is necessary to determine to what extent the skeleton regulates energy metabolism in humans.

An ELISA-based method to quantify osteocalcin carboxylation in mice

Osteocalcin was recently identified as an osteoblast-secreted hormone regulating insulin secretion and sensitivity. In mice and humans, osteocalcin can be present in the serum in carboxylated or undercarboxylated forms and it has been shown that it is the undercarboxylated form of osteocalcin which acts as a hormone. The study of osteocalcin different circulating forms in mouse serum, however, has been hampered by the absence of quantitative methodology. Here we described a triple enzyme-linked immunosorbent assay (ELISA) system for quantification of mouse total, carboxylated and uncarboxylated osteocalcin. That carboxylation of osteocalcin was decreased in mouse osteoblasts cultures treated with warfarin, an inhibitor of carboxylation validated this assay. This ELISA could also detect elevated levels of undercarboxylated osteocalcin in the serum of mice treated with warfarin and in the serum of Esp -/- mice, a mouse model known to have more undercarboxylated, i.e., active osteocalcin. These results show that this new ELISA system is a reliable method to assess carboxylation status of osteocalcin in cell culture supernatants as well as in mouse serum. Its use should facilitate the analysis of culture system or mouse model in which the hormonal activity of osteocalcin needs to be evaluated.

Extracellular fatty acid synthase: a possible surrogate biomarker of insulin resistance

CONTEXT

Circulating fatty acid synthase (FASN) is a biomarker of metabolically demanding human diseases. The aim of this study was to determine whether circulating FASN could be a biomarker of overnutrition-induced metabolic stress and insulin resistance in common metabolic disorders.

RESEARCH DESIGN AND METHODS

Circulating FASN was evaluated in two cross-sectional studies in association with insulin sensitivity and in four longitudinal studies investigating the effect of diet- and surgery-induced weight loss, physical training, and adipose tissue expansion using peroxisome proliferator-activated receptor agonist rosiglitazone on circulating FASN.

RESULTS

Age- and BMI-adjusted FASN concentrations were significantly increased in association with obesity-induced insulin resistance in two independent cohorts. Both visceral and subcutaneous FASN expression and protein levels correlated inversely with extracellular circulating FASN (P = -0.63; P < 0.0001), suggesting that circulating FASN is linked to depletion of intracellular FASN. Improved insulin sensitivity induced by therapeutic strategies that decreased fat mass (diet induced, surgery induced, or physical training) all led to decreased FASN levels in blood (P values between 0.02 and 0.04). To discriminate whether this was an effect related to insulin sensitization, we also investigated the effects of rosiglitazone. Rosiglitazone did not lead to significant changes in circulating FASN concentration.

CONCLUSIONS

Our results suggest that circulating FASN is a biomarker of overnutrition-induced insulin resistance that could provide diagnostic and prognostic advantages by providing insights on the individualized metabolic stress.

Association analysis of genetic polymorphisms and potential interaction of the osteocalcin (BGP) and ER-alpha genes with body mass index (BMI) in premenopausal Chinese women

AIM

To investigate whether estrogen receptor alpha (ER-alpha) PvuII and osteocalcin (also known as bone Gla protein, or BGP) HindIII genetic polymorphisms and their potential interactions are associated with body mass index (BMI) variation.

METHODS

Data on BMI and ER-alpha PvuII and BGP HindIII genotypes were obtained from 328 healthy premenopausal Chinese women in east China. The study subjects were unrelated, at least 21 years old (mean age of 33.2+/-5.9 years), and had an average BMI of 21.58+/-2.59. All subjects were genotyped at the ER-alpha PvuII and BGP HindIII loci using polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP).

RESULTS

The BGP HindIII genotypes were significantly associated with BMI (P=0.003). Carriers of the HH and Hh genotypes had approximately 2.73% and 1.27% higher BMI than those of the hh genotype, respectively. In contrast, the ER-alpha PvuII polymorphism was not significantly associated with BMI (P=0.454). In addition, there was no evidence of potential interactions between the ER-alpha and BGP genes in our subjects (P>or=0.013).

CONCLUSION

The HindIII polymorphism of the BGP gene, but not the PvuII polymorphism of the ER-alpha gene or their potential interaction, was associated with BMI in premenopausal Chinese women.

Carboxylation of osteocalcin affects its association with metabolic parameters in healthy children

OBJECTIVE Osteocalcin (OC), a bone-derived protein, was recently shown to regulate metabolic pathways in mice. Undercarboxylated OC (ucOC), but not carboxylated OC (cOC), increases adiponectin and insulin secretion. It is unclear if carboxylation of OC affects its association with metabolic parameters in humans. RESEARCH DESIGN AND METHODS The associations between ucOC, cOC, total and high-molecular-weight (HMW) adiponectin, and insulin secretion (homeostasis model assessment [HOMA]-beta) were investigated in a population-based sample of healthy prepubertal children (n = 103; 49 boys and 54 girls). RESULTS Weight-dependent associations were observed between the different forms of OC and metabolic parameters. Higher cOC was related to lower HMW adiponectin (with a stronger association in leaner children; P < 0.001). Higher ucOC-to-cOC ratio was associated with higher HOMA-beta (P < 0.01) in leaner children and associated with higher HMW adiponectin (P < 0.001) in heavier children. CONCLUSIONS In a weight-dependent manner, cOC and the proportion of ucOC are differentially related to HMW adiponectin and insulin secretion in healthy children.

Serum markers of bone turnover are increased by modest weight loss with or without weight-bearing exercise in overweight premenopausal women

Weight loss improves metabolic fitness and reduces morbidity and mortality; however, weight reduction also reduces bone mineral density (BMD) and increases bone turnover. Weight-bearing aerobic exercise may preserve bone mass and maintain normal bone turnover during weight reduction. We investigated the impact of weight-bearing and nonweight-bearing exercise on serum markers of bone formation and breakdown during short-term, modest weight loss in overweight premenopausal women. Subjects (n = 36) were assigned to 1 of 3 weight-loss interventions designed to produce a 5% reduction in body weight over 6 weeks: (i) energy restriction only (n = 11; DIET); (ii) energy restriction plus nonweight-bearing exercise (n = 12, CYCLE); or (iii) energy restriction plus weight-bearing exercise (n = 13, RUN). Bone turnover markers were measured in serum collected at baseline and after weight loss. All groups achieved a ~5% reduction in body weight (DIET = 5.2%; CYCLE = 5.0%; RUN = 4.7%). Osteocalcin (OC) and C-terminal telopeptide of type I collagen (CTX) increased with weight loss in all 3 groups (p < 0.05), whereas bone alkaline phosphatase was unaltered by the weight-loss interventions. At baseline, OC and CTX were positively correlated (r = 0.36, p = 0.03), but the strength of this association was diminished (r = 0.30, p = 0.06) after weight loss. Modest weight loss, regardless of method, resulted in a significant increase in both OC and CTX. Low-impact, weight-bearing exercise had no effect on serum markers of bone formation or resorption in premenopausal women during weight loss. Future studies that examine the effects of high-impact, weight-bearing activity on bone turnover and BMD during weight loss are warranted.

A new link between skeleton, obesity and insulin resistance: relationships between osteocalcin, leptin and insulin resistance in obese children before and after weight loss

BACKGROUND

The skeleton is regarded recently as an endocrine organ that affects energy metabolism. However, there are very limited data available concerning the relationships between the osteoblast-derived hormone osteocalcin, weight status, adiponectin and leptin in obese humans, especially in children.

METHODS

We analyzed osteocalcin, adiponectin, leptin and insulin resistance (IR) index homeostasis model assessment (HOMA) in 60 obese and 19 age- and gender-matched normal weight children. Furthermore, these parameters were determined in 60 obese children after participating in an outpatient 1-year lifestyle intervention based on exercise, behavior and nutrition therapy.

RESULTS

Sixty obese children had significantly lower osteocalcin levels (26.8+/-0.8 ng ml(-1)) than 19 normal weight controls (32.2+/-2.3 ng ml(-1)). Boys (29.9+/-1.1 ng ml(-1)) showed significantly (P=0.046) higher osteocalcin levels compared with girls (26.4+/-1.2 ng ml(-1)). In stepwise multiple linear regression analysis adjusted for age, gender and pubertal stage, osteocalcin was significantly negatively related to leptin and HOMA, but not to adiponectin. Changes of osteocalcin in the course of 1 year correlated significantly negatively with changes of IR index HOMA (r=-0.25), standard deviation score-body mass index (SDS-BMI) (r=-0.33) and leptin (r=-0.50). Substantial weight loss in 29 obese children led to a significant increase in osteocalcin and a significant decrease in leptin and HOMA. In 31 obese children without substantial weight loss, osteocalcin levels did not change significantly in the course of 1 year.

CONCLUSION

Osteocalcin levels were lower in obese children and were related to IR and leptin both in cross-sectional and longitudinal analyses. Therefore, osteocalcin might be a new promising link between obesity and IR.

Osteocalcin is related to enhanced insulin secretion in gestational diabetes mellitus

OBJECTIVE

There is growing evidence that osteocalcin, an osteoblast-derived protein locally acting on bone formation, can increase insulin secretion as well as insulin sensitivity and thus prevent the development of obesity and diabetes in experimental animals. In humans, osteocalcin has been reported to be decreased in patients with type 2 diabetes. Because gestational diabetes mellitus (GDM) can serve as a model of pre-type 2 diabetes, the aim of this study was to investigate osteocalcin in GDM.

RESEARCH DESIGN AND METHODS

Osteocalcin measurement and an oral glucose tolerance test were performed in 78 pregnant women (26 women had GDM and 52 women had normal glucose tolerance [NGT] during pregnancy; women were matched for age and BMI) and in 34 women postpartum.

RESULTS

During pregnancy osteocalcin was significantly higher in the women with GDM than in the women with NGT (15.6 +/- 6.4 vs. 12.6 +/- 4.0 ng/ml; P < 0.015), whereas no difference was observed between the two groups at 12 weeks postpartum (36.2 +/- 10.2 vs. 36.2 +/- 13.0 ng/ml), when osteocalcin was found to be increased compared with the level in the pregnant state in all women (+145 +/- 102% in GDM vs. +187 +/- 119% in NGT; P < 0.0001). Moreover, osteocalcin showed a significant correlation with basal and total insulin secretion in the whole study group (R = 0.3, P < 0.01).

CONCLUSIONS

In GDM osteocalcin was higher and thus less restrained than in women with NGT during pregnancy and furthermore correlated with insulin secretion parameters. Therefore, it could be hypothesized that osteocalcin can enhance insulin secretion in insulin-resistant states; alternatively an effect of hyperinsulinemia on osteocalcin secretion cannot be excluded.