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

Energy metabolism and the skeleton: Reciprocal interplay

The relation between bone remodelling and energy expenditure is an intriguing, and yet unexplained, challenge of the past ten years. In fact, it was only in the last few years that the skeleton was found to function, not only in its obvious roles of body support and protection, but also as an important part of the endocrine system. In particular, bone produces different hormones, like osteocalcin (OC), which influences energy expenditure in humans. The undercarboxylated form of OC has a reduced affinity for hydroxyapatite; hence it enters the systemic circulation more easily and exerts its metabolic functions for the proliferation of pancreatic β-cells, insulin secretion, sensitivity, and glucose tolerance. Leptin, a hormone synthesized by adipocytes, also has an effect on both bone remodelling and energy expenditure; in fact it inhibits appetite through hypothalamic influence and, in bone, stimulates osteoblastic differentiation and inhibits apoptosis. Leptin and serotonin exert opposite influences on bone mass accrual, but several features suggest that they might operate in the same pathway through a sympathetic tone. Serotonin, in fact, acts via two opposite pathways in controlling bone remodelling: central and peripheral. Serotonin product by the gastrointestinal tract (95%) augments bone formation by osteoblast, whereas brain-derived serotonin influences low bone mineral density and its decrease leads to an increase in bone resorption parameters. Finally, amylin (AMY) acts as a hormone that alters physiological responses related to feeding, and plays a role as a growth factor in bone. In vitro AMY stimulates the proliferation of osteoblasts, and osteoclast differentiation. Here we summarize the evidence that links energy expenditure and bone remodelling, with particular regard to humans.

Reduced serum osteocalcin concentrations are associated with type 2 diabetes mellitus and the metabolic syndrome components in postmenopausal women: the crosstalk between bone and energy metabolism

Although it has been shown that osteocalcin functions as a hormone in the regulation of glucose metabolism and fat mass, no population-based study to date has addressed serum osteocalcin levels in relation to energy metabolism concurrent with bone metabolism in postmenopausal women. In a population-based study, cardiovascular risk factors, high-sensitivity C-reactive protein (hs-CRP), osteoprotegerin, receptor activator of nuclear factor-κB ligand, osteocalcin, CrossLaps, alkaline phosphatase, and bone mineral density (BMD) at the lumbar spine (L2-L4) and the proximal femur were measured in 382 Iranian postmenopausal women. In multiple logistic regression analysis, lower osteocalcin and CrossLaps levels were associated with a higher odds ratio (OR) of having type 2 diabetes mellitus when adjustments were made for age, hs-CRP, cardiovascular risk factors, BMD, and markers of bone metabolism [OR 5.17, CI (2.66-10.04), p < 0.0001 and OR 2.51, CI (1.37-4.61), p = 0.003, respectively]. However, lower alkaline phosphatase levels were associated with a lower OR of having type 2 diabetes mellitus [OR 0.28, CI (0.15-0.52), p < 0.0001] in regression analysis. No significant difference was found between serum osteocalcin levels of those with and without metabolic syndrome. Among the metabolic syndrome components, low osteocalcin levels had significant associations with elevated blood glucose [OR 1.89, CI (1.16-3.07), p = 0.010] and elevated waist circumference [OR 2.53, CI (1.13-5.67), p = 0.024] in multivariate analyses. In conclusion, serum osteocalcin was independently associated with glucose intolerance and abdominal obesity as the components of metabolic syndrome and type 2 diabetes mellitus in postmenopausal women. Since CrossLaps and alkaline phosphatase levels were independently associated with the presence of type 2 diabetes mellitus, the unique contribution of osteocalcin in glucose metabolism could not be concluded.

Circulating osteocalcin level is not associated with incident type 2 diabetes in middle-aged male subjects: mean 8.4-year retrospective follow-up study

OBJECTIVE

Recent human studies suggested that serum osteocalcin is associated with the cross-talk between bone and energy metabolism. The aim of this study was to determine whether serum osteocalcin level is independently associated with the development of type 2 diabetes.

RESEARCH DESIGN AND METHODS

A retrospective cohort study was performed of 1,229 nondiabetic men, aged 25-60 years, who were recruited from the Health Promotion Center, Samsung Medical Center, between January 1997 and December 1997. They were followed regularly at the center on an out-patient basis and during hospitalization for a mean of 8.4 years, and the development of type 2 diabetes was determined.

RESULTS

In the baseline analysis, BMI, body fat percentage, triglyceride, homeostasis model assessment of insulin resistance value, and plasminogen activator inhibitor-1 levels varied inversely with the osteocalcin tertiles, and serum high-density lipoprotein cholesterol levels increased with the osteocalcin tertiles. However, no differences were observed in fasting glucose and glycated hemoglobin levels across the osteocalcin tertiles. Incident type 2 diabetes occurred in 90 (7.3%) of the study subjects. In Cox proportional hazards models, however, no statistical differences in the development of type 2 diabetes across the osteocalcin tertiles were evident after adjustment of other risk factors for incident diabetes.

CONCLUSIONS

Despite baseline associations with favorable metabolic parameters, the serum osteocalcin level was not associated with the development of type 2 diabetes in middle-aged males.

A reduced serum level of total osteocalcin in men predicts the development of diabetes in a long-term follow-up cohort

BACKGROUND

Osteocalcin (OC), an osteoblast-specific protein, has been demonstrated to affect glucose metabolism in both animals and humans. Studies in animals have shown an effect of undercarboxylated OC (ucOC) on beta-cell proliferation and insulin resistance. It remains unclear whether OC is associated with the future development of diabetes in humans, as well as the relative importance of ucOC vs OC.

OBJECTIVE

The aim of this study was to examine serum OC and its post-translational forms as potential biomarkers for future the development of type 2 diabetes.

METHODS

This was a nested case-control study using data from the Electricity Generating Authority of Thailand (EGAT). We identified 63 men without diabetes in the exploratory cohort at baseline who developed type 2 diabetes (DM) during the 10-year follow-up period from 1998-2008, and also 63 men age- and BMI-matched for a non-diabetes control group (non-DM). Serum N-mid OC and ucOC were measured in baseline blood samples. Logistic regression models were used to explore and identify baseline factors, including OC and ucOC, that predicted the subsequent development of diabetes.

RESULTS

The mean age and BMI were similar in both non-DM and DM groups (47·2 ± 0·5 vs 47·8 ± 0·8 years and 25·2 ± 0·5 vs 25·9 ± 0·5 kg/m(2) , respectively). Only baseline mean serum N-mid OC (15·2 ± 0·5 vs 13·0 ± 0·5 μg/l, P < 0·05) and fasting plasma glucose (4·92 ± 0·04 vs 5·28 ± 0·07 mmol/l, P < 0·05) were significantly different between the two groups. Multiple logistic regression analysis showed that baseline serum N-mid OC and glucose, but not ucOC, were independent risk factors for the development of diabetes in this long-term study cohort.

CONCLUSIONS

Circulating total OC is associated with incident diabetes in men. Further studies to evaluate the potential utility of OC as a biomarker to predict the development of type 2 diabetes are warranted.

Post-menopausal women with rheumatoid arthritis who are treated with raloxifene or alendronate or glucocorticoids have lower serum undercarboxylated osteocalcin levels

BACKGROUND

Serum undercarboxylated osteocalcin (ucOC) is a biochemical bone marker of vitamin K insufficiency. It had been reported that bone resorption inhibitors tend to decrease the serum ucOC level in patients with primary osteoporosis. In rheumatoid arthritis (RA) patients, these results have never been reported.

AIM

We investigated risk factors which could change serum ucOC level in post-menopausal women with RA (no.=100).

SUBJECTS AND METHODS

Twenty patients received no bone resorption inhibitor (control), 30 received raloxifene (RLX), while 50 received alendronate (ALN). This cross-sectional study was limited to patients with low RA disease activity (Disease Activity Score-28 ≤3.2). We measured serum ucOC, and the data were analyzed by multivariable analysis, including ucOC and the other variables.

RESULTS

Scheffe's F test demonstrated a significant difference in serum ucOC levels between controls and the RLX group (p<0.01), and between controls and the ALN group (p<0.01). Serum ucOC levels were low in both treated groups. An adjusted multivariate analysis was performed for the variables: bone resorption inhibitor use, serum alkaline phosphatase, glucocorticoid dose, age, estimated glomerular filtration rate and matrix metalloproteinase 3. As a result, serum ucOC inversely correlated with bone resorption inhibitor use (p<0.01) and oral glucocorticoid dose (p<0.01), which were independent risk factors of lowering ucOC.

CONCLUSIONS

Bone resorption inhibitors and glucocorticoids were independent risk factors for lowering serum ucOC levels in post-menopausal RA patients.

Determinants of undercarboxylated and carboxylated osteocalcin concentrations in type 1 diabetes

To determine whether undercarboxylated osteocalcin (UC-OC) or gamma-carboxyglutamic-carboxylated-type osteocalcin (GLA-OC) concentrations deviate from normal in type 1 diabetes (T1D), serum levels were compared between 115 subjects with T1D and 55 age-matched healthy controls. UC-OC and GLA-OC concentrations were similar between groups; however, in T1D, UC-OC correlated positively with markers of insulin exposure, either endogenously produced or exogenously administered.

INTRODUCTION

A study was conducted to determine whether dysregulation of circulating concentrations of UC-OC or GLA-OC occurs in patients with type 1 diabetes, a condition of insulin deficiency without insulin resistance.

METHODS

We measured serum concentrations of UC-OC and GLA-OC in 115 subjects with T1D, ages 14-40 years, and in 55 age-matched healthy control subjects. Relationships between UC-OC and GLA-OC concentrations and patient characteristics (gender and age), indices of glycemic control (hemoglobin A1c (HbA1c), fasting plasma glucose, C-peptide concentration, 3-day average glucose measured by a continuous glucose sensor, total daily insulin dose) and circulating indices of skeletal homeostasis (total calcium, 25-OH vitamin D, parathyroid hormone, insulin-like growth factor 1 (IGF-1), type 1 collagen degradation fragments (CTX), adiponectin, leptin) were examined. Between group differences in the concentrations of UC-OC and GLA-OC were the main outcome measures.

RESULTS

Although adiponectin levels were higher in the T1D group, between-group comparisons did not reveal statistically significant differences in concentration of UC-OC, GLA-OC, CTX or leptin between the T1D and control populations. Instead, by multivariate regression modeling, UC-OC was correlated with younger age (p < 0.001), higher CTX (p < 0.001), lower HbA1c (p = 0.013), and higher IGF-1 (p = 0.086). Moreover, within the T1D subgroup, UC-OC was positively correlated with C-peptide/glucose ratio (reflecting endogenous insulin secretion), with IGF-1 (reflecting intra-portal insulin sufficiency), and with total daily insulin dose.

CONCLUSIONS

In T1D, UC-OC appears to correlate positively with markers of insulin exposure, either endogenously produced or exogenously administered.

The endocrine role of the skeleton: background and clinical evidence

Based on the observation that diabetes, obesity, and hypogonadism influence bone metabolism, the existence of a feedback loop and a common regulation was postulated and an endocrine role ascribed to the skeleton. In the first part of this review, two pathways are described whereby adipose tissue acts on bone mass. In the first, leptin activates the sympathetic nervous system via serotonin and diminishes bone mass accrual. The second pathway functions via the activation of CART (CARTPT) and inhibits bone resorption. The first pathway leads to a decrease in bioactivity of the osteoblast-produced hormone osteocalcin (OC) (part 2). In its undercarboxylated form, OC acts on the three targets pancreas, adipose tissue, and gonads (part 3) and thereby causes an increase in insulin secretion and sensitivity, β-cell proliferation, and male fertility. Insulin (part 4) is part of a recently discovered regulatory feedback loop between pancreas and osteoblasts. It is a strong counterplayer of leptin as it causes a decrease in OPG expression and enhances bone resorption and OC decarboxylation. Numerous clinical studies (part 5) have shown associations of total and undercarboxylated OC and markers of energy metabolism. Interventional studies, to date only performed in murine models, have shown positive effects of OC administration on energy metabolism. Whether bone tissue has an even further-reaching endocrine role remains to be elucidated.

Osteocalcin and vitamin D status are inversely associated with homeostatic model assessment of insulin resistance in Canadian Aboriginal and white women: the First Nations Bone Health Study

OBJECTIVE

Osteocalcin, a protein synthesized by osteoblasts, and vitamin D status have independently been implicated in energy metabolism and glucose regulation. This study was conducted to simultaneously explore the relationships among osteocalcin, vitamin D status and indicators of glucose metabolism and adiposity in a mixed-ethnicity cohort of adult women.

DESIGN

Cross-sectional.

METHODS

Aboriginal and white women (n=368) over 25 years of age (45.3±13.6 years) were studied for measures of osteocalcin and 25-hydroxy vitamin D [25(OH)D] plus glucose metabolism including glucose, insulin, C-peptide, hemoglobin A1c (HbA1c) and homeostatic model assessment of insulin resistance (HOMA-IR). Measures of adiposity included body mass index (BMI) plus total body fat and trunk fat from dual-energy X-ray absorptiometry.

RESULTS

Aboriginal women had higher BMI, fat and markers of dysglycemia. Osteocalcin was not different between groups, but 25(OH)D was lower in Aboriginal women. Osteocalcin was inversely related to all five parameters of glucose metabolism, whereas 25(OH)D was inversely related to insulin, C-peptide and HOMA-IR. After accounting for age, ethnicity or adiposity using regression analyses, glucose, HbA1c and HOMA-IR were inversely related to both osteocalcin and 25(OH)D. However, only 25(OH)D was inversely related to C-peptide, and neither osteocalcin nor 25(OH)D was related to insulin.

CONCLUSIONS

These data from a unique mixed Aboriginal and white population suggest that both vitamin D and osteocalcin are involved in glucose control.

Osteoid osteoma is an osteocalcinoma affecting glucose metabolism

Osteocalcin is a hormone secreted by osteoblasts, which regulates energy metabolism by increasing β-cell proliferation, insulin secretion, insulin sensitivity, and energy expenditure. This has been demonstrated in mice, but to date, the evidence implicating osteocalcin in the regulation of energy metabolism in humans are indirect. To address this question more directly, we asked whether a benign osteoblastic tumor, such as osteoma osteoid in young adults, may secrete osteocalcin. The study was designed to assess the effect of surgical resection of osteoid osteoma on osteocalcin and blood glucose levels in comparison with patients undergoing knee surgery and healthy volunteers. Blood collections were performed the day of surgery and the following morning after overnight fasting. Patients and controls were recruited in the orthopedic surgery department of New York Presbiterian Hospital, NY-USA and Hospices Civils de Lyon, France. Seven young males were included in the study: two had osteoid osteoma, two underwent knee surgery, and three were healthy volunteers. After resection of the osteoid osteomas, we observed a decrease of osteocalcin by 62% and 30% from the initial levels. Simultaneously, blood glucose increased respectively by 32% and 15%. Bone turnover markers were not affected. This case study shows for the first time that osteocalcin in humans affects blood glucose level. This study also suggests that ostoid osteoma may be considered, at least in part, as an osteocalcinoma.

Bone metabolism and fracture risk in type 2 diabetes mellitus

Osteoporosis and type 2 diabetes mellitus (T2DM), both prevalent in aging and westernized societies, adversely affect the health of elderly people by causing fractures and vascular complications, respectively. Recent experimental and clinical studies show that the disorders are etiologically related through the actions of osteocalcin and adiponectin. Meta-analyses of multiple clinical studies show that the hip fracture risk of T2DM patients is increased 1.4-1.7-fold compared with non-DM controls, even though the patients' bone mineral density (BMD) is not diminished. Vertebral fracture risk of the T2DM patients is also increased, and BMD measurement is not sensitive enough to assess this risk. These findings suggest that bone fragility in T2DM patients depends on bone quality deterioration rather than bone mass reduction. Surrogate markers are therefore needed to supplement the partial effectiveness of BMD testing in assessing the fracture risk of the T2DM patients. Markers related to advanced glycation end products may be candidates. These substances modulate bone quality in DM. Until research establishes the usefulness of surrogate markers, physicians should assess fracture risk in T2DM patients not only by measuring the BMD, but also by taking a fracture history and evaluating prior vertebral fractures using spinal X-rays.

Intermittent injections of osteocalcin improve glucose metabolism and prevent type 2 diabetes in mice

The uncarboxylated form of the osteoblast-specific secreted molecule osteocalcin is a hormone favoring glucose handling and increasing energy expenditure. As a result, the absence of osteocalcin leads to glucose intolerance in mice, while genetically modified mice with an increase in uncarboxylated osteocalcin are protected from type 2 diabetes and obesity. Here, we tested in the mouse the therapeutic potential of intermittent administration of osteocalcin. We found that daily injections of osteocalcin at either 3 or 30 ng/g/day significantly improved glucose tolerance and insulin sensitivity in mice fed a normal diet. This was attributable, in part, to an increase in both β-cell mass and insulin secretion. When mice were fed a high-fat diet (HFD), daily injections of osteocalcin partially restored insulin sensitivity and glucose tolerance. Moreover, mice treated with intermittent osteocalcin injections displayed additional mitochondria in their skeletal muscle, had increased energy expenditure and were protected from diet-induced obesity. Finally, the hepatic steatosis induced by the HFD was completely rescued in mice receiving osteocalcin daily. Overall, these results provide evidence that daily injections of osteocalcin can improve glucose handling and prevent the development of type 2 diabetes.

Association between vitamin K intake from fermented soybeans, natto, and bone mineral density in elderly Japanese men: the Fujiwara-kyo Osteoporosis Risk in Men (FORMEN) study

SUMMARY

A cross-sectional analysis of 1,662 community dwelling elderly Japanese men suggested that habitual natto intake was significantly associated with higher bone mineral density (BMD). When adjustment was made for undercarboxylated osteocalcin levels, this association was insignificant, showing the natto-bone association to be primarily mediated by vitamin K.

INTRODUCTION

Low vitamin K intake is associated with an increased risk of hip fracture, but reports have been inconsistent on its effect on BMD. Our first aim was to examine the association between BMD and intake of fermented soybeans, natto, which contain vitamin K1 (20 μg/pack) and K2 (380 μg/pack). Our second aim was to examine the association between undercarboxylated osteocalcin (ucOC), a biomarker of vitamin K intake, and BMD to evaluate the role of vitamin K in this association.

METHODS

Of the Japanese men aged ≥65 years who participated in the baseline survey of the Fujiwara-kyo Osteoporosis Risk in Men study, 1,662 men without diseases or medications known to affect bone metabolism were examined for associations between self-reported natto intake or serum ucOC levels with lumbar spine or hip BMD.

RESULTS

The subjects with greater intake of natto showed significantly lower level of serum ucOC. Analysis after adjustment for confounding variables showed an association of greater intake of natto with both significantly higher BMD and lower risk of low BMD (T-score < -1 SD) at the total hip and femoral neck. This association became insignificant after further adjustment for ucOC level.

CONCLUSION

Habitual intake of natto was associated with a beneficial effect on bone health in elderly men, and this association is primarily due to vitamin K content of natto, although the lack of information on dietary nutrient intake, including vitamin K1 and K2, prevented us from further examining the association.

Serum undercarboxylated osteocalcin levels are inversely associated with glycemic status and insulin resistance in an elderly Japanese male population: Fujiwara-kyo Osteoporosis Risk in Men (FORMEN) Study

SUMMARY

Recent animal studies have demonstrated that undercarboxylated osteocalcin upregulates insulin secretion via osteoblast-insulin signaling. However, it remains unclear whether such a pathway exists in humans. This study showed that serum undercarboxylated osteocalcin levels were inversely associated with fasting plasma glucose, hemoglobin A(1c), and homeostasis model assessment of insulin resistance (HOMA-IR) levels in community-dwelling elderly Japanese men.

INTRODUCTION

Undercarboxylated osteocalcin (ucOC) was reported to increase insulin secretion and improve glucose tolerance via osteoblast-insulin signaling in animal-based studies. Whether this pathway also exists in humans is unknown. We aimed to clarify whether serum ucOC levels are associated with glycemic status and insulin resistance in the general Japanese population.

METHODS

We included 2,174 Japanese men (≥65 years) who were able to walk without aid from others and lived at home in four cities of Nara Prefecture. We excluded participants with a history of diseases or medications that affect bone metabolism, other than type 2 diabetes mellitus (T2DM). Fasting plasma glucose, glycated hemoglobin A(1c), and HOMA-IR levels were determined as outcome measures.

RESULTS

Of the 1,597 participants included in the analysis, both intact OC (iOC) and ucOC levels showed significant inverse correlations with all outcome measures, even after adjusting for potential confounders. Mean values of outcome measures showed a significant decreasing trend with higher quintiles of iOC or ucOC after adjusting for confounders. This trend remained significant for ucOC quintiles after further adjustment for iOC levels, but was not significant for iOC quintiles after adjusting for ucOC levels. These results were attenuated, but still apparent, after excluding participants receiving drug therapy for T2DM.

CONCLUSIONS

Levels of ucOC, but not iOC, were inversely associated with glycemic index and insulin resistance in a population of Japanese men. These findings will need to be confirmed with longitudinal studies.

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.

Change in undercarboxylated osteocalcin is associated with changes in body weight, fat mass, and adiponectin: parathyroid hormone (1-84) or alendronate therapy in postmenopausal women with osteoporosis (the PaTH study)

CONTEXT

The undercarboxylated form of the osteoblast-secreted protein osteocalcin has favorable effects on fat and glucose metabolism in mice. In human subjects, cross-sectional studies suggest a relevant association.

OBJECTIVE

We investigated whether changes in undercarboxylated osteocalcin (ucOC) during osteoporosis treatment are associated with changes in metabolic parameters.

DESIGN, SETTING, PARTICIPANTS, AND INTERVENTIONS

We measured ucOC in sera from a subset of osteoporotic postmenopausal women who were treated with PTH(1-84) or alendronate (n = 64 and n = 33, respectively) during the Parathyroid Hormone and Alendronate study.

MAIN OUTCOME MEASURES

We measured serum adiponectin, leptin, and insulin and analyzed existing data on body weight, fat mass, and serum glucose concentration. Three-month changes in ucOC levels were evaluated as predictors of 12-month changes in indices of fat and glucose metabolism.

RESULTS

ucOC levels increased with PTH(1-84) and decreased with alendronate administration (P ≤ 0.01 for both treatment groups). Three-month change in ucOC was inversely associated with 12-month changes in body weight (standardized β = -0.25, P = 0.04) and fat mass (β = -0.23, P = 0.06), after adjustment for the treatment group. Three-month change in ucOC was positively associated with a 12-month change in adiponectin (β = 0.30, P = 0.01), independent of change in fat mass. There were no interactions between treatment and change in ucOC on changes in weight, fat mass, or adiponectin.

CONCLUSIONS

PTH(1-84) increases and alendronate decreases ucOC levels. Changes in ucOC induced by PTH(1-84) and alendronate are associated with changes in metabolic indices. These associations are consistent with observations from animal models and support a role for ucOC in the skeletal regulation of energy metabolism in humans.

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.

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.

Regulation of glucose metabolism and the skeleton

Complex interactions occur among adipose tissue, the central nervous system, bone and pancreas to integrate bone remodelling, glucose, lipid and energy metabolism. Data obtained largely from the judicious use of gain-of-function and loss-of-function genetic mouse models show that leptin, an adipocyte-secreted product, indirectly inhibits bone accrual through a central pathway comprising the hypothalamus and central nervous system. Increased sympathetic output acting via β2-adrenergic receptors present in osteoblasts decreases bone formation and causes increased bone resorption. Insulin is a key molecular link between bone remodelling and energy metabolism. Insulin signalling in the osteoblasts increases bone formation and resorption as well as the release of undercarboxylated osteocalcin. An increase in the release of bone-derived undercarboxylated osteocalcin into the systemic circulation enables it to act as a circulating hormone to stimulate insulin production and secretion by pancreatic β-cells and adiponectin by adipocytes. Insulin sensitivity increases, lipolysis and fat accumulation decreases while energy expenditure increases. Whether this model of integrative physiology involving the skeleton, pancreas and adipose tissue, so elegantly demonstrated in rodents, is applicable to humans is controversial. The mouse Esp gene, encoding an intracellular tyrosine phosphatase that negatively regulates insulin signalling in osteoblasts, is a pseudogene in humans, and a homolog for the Esp gene has so far not been identified in humans. A close homologue of Esp, PTP1B, is expressed in human osteoblasts and could take the role of Esp in humans. Data available from the limited number of clinical studies do not provide a sufficient body of evidence to determine whether osteocalcin or undercarboxylated osteocalcin affects glucose metabolism in humans.

Androgens modulate osteocalcin release by human visceral adipose tissue

OBJECTIVE

Androgens inhibit adipogenic differentiation through an androgen receptor (AR)-mediated pathway, increase lipolysis and reduce lipid accumulation in adipocytes. Undercarboxylated osteocalcin (ucOCN) regulates insulin and adiponectin secretion and is released by adipose tissue (AT). Our objective was to investigate, ex vivo and in vivo, the role of androgens on osteocalcin (OCN) modulation in human AT. DesiGN, PATIENTS, SETTING: Omental AT (OAT) for in vitro study and blood samples from 91 male patients of Padova University Hospital were used.

MEASUREMENTS

Omental AT was treated with dihydrotestosterone (DHT) in presence and in absence of flutamide. cOCN and ucOCN release by AT in a simple growth medium was evaluated by ELISA. OCN, both undercarboxylated (ucOCN) and carboxylated (cOCN) forms, was measured in serum by ELISA.

RESULTS

After 24-h DHT stimulation, the release of both cOCN and ucOCN by OAT was statistically increased (P < 0·05). Co-incubation with flutamide blunted OCN production. Overweight and obese patients had lower total and free testosterone (T), associated with lower ucOCN and ucOCN/OCN ratio. Free T was negatively correlated to BMI (ρ = -0·706, P < 0·05) and positively correlated to ucOCN/OCN ratio (ρ = 0·223, P < 0·05).

CONCLUSIONS

Our data suggest that androgens modulate OCN release by OAT in vitro. In addition to the anti-adipogenic role of androgens, they support a novel mechanism by which androgens could exert a protective effect in energy metabolism. This hypothesis appears even more significant considering that sexual hormones' levels are greatly altered in obesity and that AT is both highly involved in their clearance and able to produce OCN.

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.

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.

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.

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.

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.

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.

Effect of phylloquinone supplementation on glucose homeostasis in humans

BACKGROUND

Under-γ-carboxylated osteocalcin (ucOC) increases insulin secretion and decreases glucose concentrations in mice.

OBJECTIVE

We determined whether changes in ucOC concentrations in humans were associated with changes in insulin and glucose concentrations.

DESIGN

Twenty-one community-dwelling postmenopausal women received 1 mg phylloquinone daily for 12 mo (experimental group), and 21 subjects were treated with a placebo during the same period (control group). Total serum osteocalcin, ucOC, glucose, and insulin concentrations were measured before and 6 and 12 mo after treatment. The homeostasis model assessment of insulin resistance (HOMA-IR) was calculated and correlated with ucOC concentrations.

RESULTS

Before administration of the placebo or phylloquinone, total osteocalcin, ucOC, glucose, and insulin concentrations and HOMA-IR (1.24 ± 0.15 for the control group compared with 1.93 ± 0.37 for the experimental group) did not differ. After treatment, total osteocalcin concentrations were similar at 6 and 12 mo. At 6 mo, serum ucOC concentrations in the experimental group were 0.96 ± 0.08 ng/mL compared with 2.94 ± 0.27 ng/mL in the control group (P < 0.001). At 12 mo, serum ucOC concentrations were 0.92 ± 0.09 ng/mL and 3.13 ± 0.26 ng/mL (P < 0.001) in experimental and control groups, respectively. Despite a decrease of ≈200% in ucOC concentrations, HOMA-IR was similar in the 2 groups at 6 and 12 mo (at 6 mo, HOMA-IR was 2.24 ± 0.54 and 1.52 ± 0.23 in the experimental and control groups, respectively; at 12 mo, HOMA-IR was 2.13 ± 0.38 and 1.47 ± 0.22 in the experimental and control groups, respectively; P = NS).

CONCLUSIONS

In postmenopausal women, phylloquinone administration is not associated with changes in insulin secretion and action despite reductions in ucOC concentrations. Changes in ucOC concentrations do not alter glucose metabolism in women. This trial was registered at clinicaltrials.gov as NCT00062595.

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.

Inflammation as death or life signal in diabetic fracture healing

Increased apoptosis of chondrocytes and osteoblasts and prolonged survival of osteoclasts lead to early destruction of callus tissue and impair bone remodeling in fracture healing of diabetic patients. Diabetes is accompanied by an increased inflammatory state, reactive oxygen species (ROS) generation and accumulation of advanced glycation end products (AGEs), a heterogenous group of toxic metabolites that can induce inflammation. Prolonged hyperglycemia and insulin resistance correlate with increased apoptosis rate and, accordingly, the proapoptotic role of several inflammatory mediators, ROS and AGEs has been also documented. In this review we summarize the most recent reports supporting the idea that inflammatory signaling increases chondrocyte and osteoblast death and prolongs osteoclast survival, resulting in impaired bone regeneration in diabetes. Antagonising inflammatory signal pathways and solution of inflammation may deserve greater attention in the management of diabetic fracture healing.

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.