Associations of total and undercarboxylated osteocalcin with peripheral and hepatic insulin sensitivity and β-cell function in overweight adults

Impact of Osteocalcin on Glycemic Regulation and Insulin Sensitivity in Type 2 Diabetes Mellitus Patients

Background Type 2 diabetes mellitus (T2DM) is a worldwide health issue impacting millions of individuals. In recent years, bone has been identified as an endocrine organ that regulates glucose metabolism by the release of osteocalcin, an osteoblast-specific hormone, which affects fat accumulation and blood glucose levels. Osteocalcin has been associated with insulin sensitivity and glucose control. Objective The study investigates the relationship between circulating osteocalcin levels with glycemic control parameters and insulin resistance in T2DM patients. Methods A total of 234 subjects were recruited, including T2DM patients (n=117) and age-sex-matched controls (n=117). Fasting blood samples were collected to measure fasting blood sugar (FBS), insulin, glycated hemoglobin (HbA1c), and osteocalcin levels. Osteocalcin levels were determined using an enzyme-linked immunosorbent assay. Insulin resistance was calculated using the Homeostatic Model Assessment for Insulin Resistance (HOMA-IR). Results The levels of osteocalcin in T2DM patients were significantly lower (7.07 ± 3.80 ng/mL) than in healthy controls (20.41 ± 13.50 ng/mL, p<0.0001). A significant negative correlation was observed between osteocalcin and HbA1c (r=-0.710, p<0.01), as well as between osteocalcin and FBS (r=-0.676, p<0.01). T2DM patients also showed significantly higher insulin resistance, as evidenced by their elevated HOMA-IR scores (4.39 ± 1.95 vs. 3.62 ± 1.82, p=0.002). There was a negative correlation between osteocalcin and HOMA-IR (r=-0.324, p=0.0001). Conclusion This study shows that osteocalcin levels are significantly reduced in patients with T2DM and demonstrate a negative correlation with HbA1c, FBS, and insulin resistance.

The Pathological Links between Adiposity and the Carpal Tunnel Syndrome

An association between obesity and carpal tunnel syndrome is found in many epidemiological studies. Therefore, there is a need to evaluate the physiopathological links that could explain the association between these two entities. Ectopic adipose tissue is responsible for metabolic syndrome and inflammation, and is a major risk factor for diabetes and cardiovascular diseases. Taking these elements into consideration, we conducted an extensive literature revision of the subject, considering as ectopic fat-related mechanisms the following: (a) the direct compression and the association with the metabolic syndrome of the fat deposition around the wrist, (b) the insulin resistance, dyslipidemia, inflammatory, and oxidative mechanisms related to the central deposition of the fat, (c) the impaired muscle contraction and metabolism related to myosteatosis. Each section presents the cellular pathways which are modified by the ectopic deposition of the adipose tissue and the impact in the pathogeny of the carpal tunnel syndrome. In conclusion, the experimental and clinical data support the epidemiological findings. Efforts to reduce the obesity epidemics will improve not only cardio-metabolic health but will reduce the burden of the disability-free life expectancy due to the carpal tunnel syndrome.

Stress Hyperglycemia and Osteocalcin in COVID-19 Critically Ill Patients on Artificial Nutrition

We aimed to study the possible association of stress hyperglycemia in COVID-19 critically ill patients with prognosis, artificial nutrition, circulating osteocalcin, and other serum markers of inflammation and compare them with non-COVID-19 patients. Fifty-two critical patients at the intensive care unit (ICU), 26 with COVID-19 and 26 non-COVID-19, were included. Glycemic control, delivery of artificial nutrition, serum osteocalcin, total and ICU stays, and mortality were recorded. Patients with COVID-19 had higher ICU stays, were on artificial nutrition for longer (p = 0.004), and needed more frequently insulin infusion therapy (p = 0.022) to control stress hyperglycemia. The need for insulin infusion therapy was associated with higher energy (p = 0.001) and glucose delivered through artificial nutrition (p = 0.040). Those patients with stress hyperglycemia showed higher ICU stays (23 ± 17 vs. 11 ± 13 days, p = 0.007). Serum osteocalcin was a good marker for hyperglycemia, as it inversely correlated with glycemia at admission in the ICU (r = -0.476, p = 0.001) and at days 2 (r = -0.409, p = 0.007) and 3 (r = -0.351, p = 0.049). In conclusion, hyperglycemia in critically ill COVID-19 patients was associated with longer ICU stays. Low circulating osteocalcin was a good marker for stress hyperglycemia.

Serum C-peptide and osteocalcin levels in children with recently diagnosed diabetes

BACKGROUND

We explored the association of C-peptide (marker of secreted insulin), proinsulin and proinsulin ⁄C-peptide ratio (PI/C) (markers of beta-cell endoplasmic reticulum [ER] stress) with undercarboxylated (uOC) and carboxylated osteocalcin (cOC) and their ratio (uOC/cOC) in children with recently diagnosed type 1 (T1D) or type 2 diabetes (T2D), and the correlation of these variables with partial remission (PR) in children with T1D.

METHODS

Demographic and clinical data of children with new-onset diabetes (n = 68; median age = 12.2 years; 33.8% non-Hispanic White, 45.6% Hispanic/Latino, 16.2% African American and 4.4% other) were collected at diagnosis and during the first (V1), second (V2) and third clinical visits at 9.0, 32.0 and 175.7 weeks, respectively. Serum proinsulin, C-peptide, uOC and cOC values were measured 7.0 weeks after diagnosis. PR was defined as insulin dose-adjusted HbA1c (IDAA1c) ≤9.

RESULTS

In children with new-onset T1D with DKA (33.3%) or T2D (29.4%), Spearman's correlation coefficient revealed a positive association between the C-peptide levels and both uOC and uOC/cOC ratio. In T1D (n = 48), both higher serum C-peptide levels and low PI:C ratio were associated with higher BMI percentile (β = 0.02, P = .001; β = -0.01, P = .02, respectively) and older age at diagnosis (β = 0.13, P = .001; β = -0.12, P = .001, respectively). Furthermore, in children with T1D, C-peptide levels at V1 correlated with IDAA1c ≤ 9 at V1 (P = .04).

CONCLUSION

C-peptide levels are associated with a higher uOC and uOC/cOC ratio in paediatric diabetes. In new-onset T1D children, older age and higher BMI were associated with lower beta-cell stress and higher preserved function, which was predictive of PR on follow-up.

The role of osteoblasts in energy homeostasis

Osteoblasts are specialized mesenchymal cells that synthesize bone matrix and coordinate the mineralization of the skeleton. These cells work in harmony with osteoclasts, which resorb bone, in a continuous cycle that occurs throughout life. The unique function of osteoblasts requires substantial amounts of energy production, particularly during states of new bone formation and remodelling. Over the last 15 years, studies have shown that osteoblasts secrete endocrine factors that integrate the metabolic requirements of bone formation with global energy balance through the regulation of insulin production, feeding behaviour and adipose tissue metabolism. In this article, we summarize the current understanding of three osteoblast-derived metabolic hormones (osteocalcin, lipocalin and sclerostin) and the clinical evidence that suggests the relevance of these pathways in humans, while also discussing the necessity of specific energy substrates (glucose, fatty acids and amino acids) to fuel bone formation and promote osteoblast differentiation.

Correlation Between Serum miR-154-5p and Osteocalcin in Males and Postmenopausal Females of Type 2 Diabetes With Different Urinary Albumin Creatinine Ratios

Purpose: To investigate the serum levels of miR-154-5p, osteocalcin (OC), and other clinical parameters in male and post-menopausal female type 2 diabetes mellitus (T2DM) patients with different urinary albumin creatinine ratio (UACR) levels and to discuss the relationship between miR-154-5p and glycolipid metabolism, bone metabolism, and different urinary albumin excretion rate in T2DM. Methods: Seven hundred thirty-eight T2DM patients were categorized into six groups, including 374 men and 364 post-menopausal women who were sub-divided into three groups based on albumin excretion that involved normal albuminuria, microalbuminuria, and large amount of albuminuria (138, 127, 109, 135, 125, and 104 cases, UACR<30, 30-300, and >300 mg/g, M1, M2, M3, F1, F2, and F3). Measurement of circulating miR-154-5p, OC, and other biochemical indicators were performed by real-time PCR, ELISA, and chemiluminescence assays in T2DM patients and in 141 M0 and 139 F0 control subjects. Results: There are few differences appeared between groups. Comparing with men, women had higher age, waist-to-hip ratio (WHR), adiponectin (ADPN), connective tissue growth factor (CTGF), UACR, procollagen type 1 N-terminal propeptide (P1NP), β-C-terminal telopeptide of type I collagen (β-CTx), OC, and miR-154-5p, but lower FPG, HOMA-IR, and HbA1c. T2DM patients with albuminuria (micro or macro) had lower bone turnover markers (P1NP, β-CTx, and OC) and adiponectin, but higher HbA1c, CTGF, and miR-154-5p. In addition, after regression analysis, UACR was positively correlated with CTGF, HbA1c, and miR-154-5p, and negatively correlated with ADPN and bone turnover markers (P1NP, β-CTx, and OC). However, OC showed a positive correlation with ADPN and other bone turnover markers (P1NP and β-CTx), but negative correlation with CTGF, UACR, and miR-154-5p in all three groups. Conclusion: These findings suggested that increased serum levels of miR-154-5p and decreased OC levels may influence osteogenesis and proteinuria in T2DM and may identify novel targets for diagnosis and treatment of diabetic kidney disease and osteoporosis.

The role of the musculoskeletal system in post-burn hypermetabolism

Burn injury results in a triad of inter-related adaptive responses: a systemic inflammatory response, a stress response, and a consequent hypermetabolic state which supports the former two. Details of what precisely triggers these responses as well as the sequence of events leading up to these responses are not clear. We review the musculoskeletal effects of burn injury to determine the precise contributions of this system in the generation and sustenance of this post-burn triad as well as the possible effects of pharmacologic intervention in the musculoskeletal response to burns on the resulting hypermetabolism. Inflammation-associated bone resorption liberates calcium, which may either prolong or intensify the systemic inflammatory response. Phosphate and magnesium liberated from bone could contribute to sustaining the increased ATP turnover in skeletal muscle that accompanies burn hypermetabolism. Reduced bone formation resulting from both pro-inflammatory cytokines and elevated endogenous glucocorticoid production results in reduced bone mass and therefore reduced osteocalcin production, which may contribute to reduced glucose uptake by skeletal muscle. Moreover, bone resorption liberates muscle catabolic factors such as transforming growth factor β, which contribute to the muscle wasting of burn hypermetabolism. Pharmacologic intervention with anti-resorptive agents early in the process preserve bone and muscle mass post-burn and future research should address the consequences for the hypermetabolic triad duration and intensity accompanying burn injury.

Lower serum osteocalcin concentrations in patients with type 2 diabetes and relationships with vascular risk factors among patients with coronary artery disease

BACKGROUND

Lower serum concentrations of the osteoblast-derived protein, osteocalcin, have been associated with poorer glycemic control, insulin resistance and atherosclerosis, and with the development of type 2 diabetes (T2DM).

METHODS

This study compares concentrations of two physiological forms of osteocalcin, carboxylated (cOCN) and uncarboxylated (unOCN), between participants with T2DM (n = 20) and age-, gender- and body mass index (BMI)-matched participants without T2DM (n = 40) among patients with coronary artery disease (CAD), and it explores relationships between osteocalcin concentrations and cardiovascular risk factors.

RESULTS

Concentrations of unOCN (2.71 ± 1.86 vs. 4.70 ± 2.03 ng/mL; t = -3.635, p = 0.001) and cOCN (8.70 ± 2.27 vs. 10.77 ± 3.69 ng/mL; t = -2.30, p = 0.025) were lower in participants with T2DM. In participants without T2DM, concentrations of cOCN were associated with fitness (VO_2Peak rho = 0.317, p = 0.047) and lower body fat (rho = -0.324, p = 0.041). In participants with T2DM, lower unOCN was associated with HbA1c (rho = -0.516, p = 0.020). Higher body mass was associated with higher unOCN (rho = 0.423, p = 0.009) in participants without T2DM, but with lower concentrations of both unOCN (rho = -0.590, p = 0.006) and cOCN (rho = -0.632, p = 0.003) in participants with T2DM.

CONCLUSION

In patients with CAD, lower osteocalcin concentrations were related to type 2 diabetes, and to adverse fitness, metabolic and obesity profiles.

Combination of Aronia, Red Ginseng, Shiitake Mushroom and Nattokinase Potentiated Insulin Secretion and Reduced Insulin Resistance with Improving Gut Microbiome Dysbiosis in Insulin Deficient Type 2 Diabetic Rats

The combination of freeze-dried aronia, red ginseng, ultraviolet-irradiated shiitake mushroom and nattokinase (AGM; 3.4:4.1:2.4:0.1) was examined to evaluate its effects on insulin resistance, insulin secretion and the gut microbiome in a non-obese type 2 diabetic animal model. Pancreatectomized (Px) rats were provided high fat diets supplemented with either (1) 0.5 g AGM (AGM-L), (2) 1 g AGM (AGM-H), (3) 1 g dextrin (control), or (4) 1 g dextrin with 120 mg metformin (positive-control) per kg body weight for 12 weeks. AGM (1 g) contained 6.22 mg cyanidin-3-galactose, 2.5 mg ginsenoside Rg3 and 244 mg β-glucan. Px rats had decreased bone mineral density in the lumbar spine and femur and lean body mass in the hip and leg compared to the normal-control and AGM-L and AGM-H prevented the decrease. Visceral fat mass was lower in the control group than the normal-control group and its decrease was smaller with AGM-L and AGM-H. HOMA-IR was lower in descending order of the control, positive-control, AGM-L, AGM-H and normal-control groups. Glucose tolerance deteriorated in the control group and was improved by AGM-L and AGM-H more than in the positive-control group. Glucose tolerance is associated with insulin resistance and insulin secretion. Insulin tolerance indicated insulin resistance was highly impaired in diabetic rats, but it was improved in the ascending order of the positive-control, AGM-L and AGM-H. Insulin secretion capacity, measured by hyperglycemic clamp, was much lower in the control group than the normal-control group and it was improved in the ascending order of the positive-control, AGM-L and AGM-H. Diabetes modulated the composition of the gut microbiome and AGM prevented the modulation of gut microbiome. In conclusion, AGM improved glucose metabolism by potentiating insulin secretion and reducing insulin resistance in insulin deficient type 2 diabetic rats. The improvement of diabetic status alleviated body composition changes and prevented changes of gut microbiome composition.

The osteoblast: Linking glucocorticoid-induced osteoporosis and hyperglycaemia? A post-hoc analysis of a randomised clinical trial

HYPOTHESIS

Glucocorticoids (GCs) induce osteoporosis predominantly by inhibiting osteoblast activity. We hypothesised that osteoblastic factors could also be linked to GC-induced adverse metabolic effects.

METHODS

We performed a post-hoc analysis of a randomised, placebo-controlled, double blind, dose-response intervention study involving 32 healthy males (age: 22 ± 3 years; BMI 22.4 ± 1.7 kg/m²) who were allocated to prednisolone (PRED) 7.5 mg once daily (n = 12), PRED 30 mg once daily (n = 12), or placebo (n = 8) for two weeks using block randomisation. Mean outcomes measures included osteocalcin, N-terminal propeptide of type 1 procollagen (P1NP) and their relation to glucose and lipid metabolism, measured by stable isotopes, before and at 2 weeks of treatment, in the fasted state and during a two-step hyperinsulinaemic clamp.

RESULTS

Osteocalcin and P1NP concentrations were dose-dependently decreased by PRED treatment (p < 0.001 both). PRED dosages dose-dependently reduced sensitivity of the liver and skeletal muscle for insulin (p < 0.001 both) and impaired suppression of lipolysis mediated by insulin (p < 0.01). In multivariate analyses, GC-induced changes in osteocalcin concentrations related to reduces hepatic insulin sensitivity (β = -0.315; p = 0.044). In addition, GC-induced changes in P1NP were negatively related to changes in insulin-mediated suppression of hepatic glucose production (r = -0.582; p = 0.001), and were positively related to insulin-stimulated glucose uptake (r = 0.638; p < 0.001). Finally, changes in PN1P were negatively related to changes in fasting hypertriglyceridemia (r = -0.499; p = 0.004) and insulin-induced suppression of lipolysis rates (r = -0.494; p = 0.006).

CONCLUSION

GC treatment alters osteoblastic function which is associated with several adverse metabolic effects of GC treatment. Future causal studies are needed to assess the specific mediator(s) by which the osteoblast alters intermediary metabolism.

CLINICAL TRIAL REGISTRATION NUMBER

ISRCTN83991850.

Undercarboxylated Osteocalcin: Experimental and Human Evidence for a Role in Glucose Homeostasis and Muscle Regulation of Insulin Sensitivity

Recent advances have indicated that osteocalcin, and in particular its undercarboxylated form (ucOC), is not only a nutritional biomarker reflective of vitamin K status and an indicator of bone health but also an active hormone that mediates glucose metabolism in experimental studies. This work has been supported by the putative identification of G protein-coupled receptor, class C, group 6, member A (GPRC6A) as a cell surface receptor for ucOC. Of note, ucOC has been associated with diabetes and with cardiovascular risk in epidemiological studies, consistent with a pathophysiological role for ucOC in vivo. Limitations of existing knowledge include uncertainty regarding the underlying mechanisms by which ucOC interacts with GPRC6A to modulate metabolic and cardiovascular outcomes, technical issues with commonly used assays for ucOC in serum, and a paucity of clinical trials to prove causation and illuminate the scope for novel health interventions. A key emerging area of research is the role of ucOC in relation to expression of GPRC6A in muscle, and whether exercise interventions may modulate metabolic outcomes favorably in part via ucOC. Further research is warranted to clarify potential direct and indirect roles for ucOC in human health and cardiometabolic diseases.

The relationship between bone turnover and insulin sensitivity and secretion: Cross-sectional and prospective data from the RISC cohort study

Bone metabolism appears to influence insulin secretion and sensitivity, and insulin promotes bone formation in animals, but similar evidence in humans is limited. The objectives of this study are to explore if bone turnover markers were associated with insulin secretion and sensitivity and to determine if bone turnover markers predict changes in insulin secretion and sensitivity. The study population encompassed 576 non-diabetic adult men with normal glucose tolerance (NGT; n=503) or impaired glucose regulation (IGR; n=73). Baseline markers of bone resorption (CTX) and formation (P1NP) were determined in the fasting state and after a 2-h hyperinsulinaemic, euglycaemic clamp. An intravenous glucose tolerance test (IVGTT) and a 2-h oral glucose tolerance test (OGTT) were performed at baseline, and the OGTT was repeated after 3years. There were no differences in bone turnover marker levels between NGT and IGR. CTX and P1NP levels decreased by 8.0% (p<0.001) and 1.9% (p<0.01) between baseline and steady-state during the clamp. Fasting plasma glucose was inversely associated with CTX and P1NP both before and after adjustment for recruitment centre, age, BMI, smoking and physical activity. However, baseline bone turnover markers were neither associated with insulin sensitivity (assessed using hyperinsulinaemic euglycaemic clamp and OGTT) nor with insulin secretion capacity (based on IVGTT and OGTT) at baseline or at follow-up. Although inverse associations between fasting glucose and markers of bone turnover were identified, this study cannot support an association between insulin secretion and sensitivity in healthy, non-diabetic men.

[Osteocalcin and glucose metabolism: assessment of human studies]

Osteocalcin, a protein secreted by osteoblasts, is recognized as a biomarker of bone mineralization. Besides, animal experimental studies have shown that osteocalcin could play an important role in glucose metabolism. Over the course of the last decade, this theory has been investigated in several human studies. Most of the results obtained from these reports support a positive correlation between total and undercarboxylated osteocalcin and insulin secretion/sensitivity. Studies also corroborate a reverse association with glycemic parameters such as fasting glucose and glycated hemoglobin. Nevertheless, because most of the studies published are observational, it is not possible to yet confirm a direct cause-effect relationship. Research in the field will surely contribute to the development of new pharmacological strategies for the treatment of endocrine diseases.

Associations among osteocalcin, leptin and metabolic health in children ages 9-13 years in the United States

Background

This study aimed to investigate the relationships among osteocalcin, leptin and metabolic health outcomes in children ages 9–13 years.

Methods

This was a cross-sectional analysis of baseline data from 161 boys and 157 girls (ages 9–13 years) who previously participated in a double-blinded randomized placebo controlled trial of vitamin D supplementation. Relationships among fasting serum total osteocalcin (tOC), undercarboxylated osteocalcin (ucOC), leptin, and metabolic health outcomes were analyzed.

Results

Approximately 52% of study participants were obese based on percent body fat cutoffs (>25% for boys and >32% for girls) and about 5% had fasting serum glucose within the prediabetic range (i.e. 100 to 125 mg/dL). Serum tOC was not correlated with leptin, glucose, insulin, HOMA-IR, or HOMA-β after adjusting for percent body fat. However, serum ucOC negatively correlated with leptin (partial r = −0.16; p = 0.04) and glucose (partial r = −0.16; p = 0.04) after adjustment for percent body fat. Leptin was a positive predictor of insulin, glucose, HOMA-IR, and HOMA-β after adjusting for age, sex and percent body fat (all p < 0.001).

Conclusions

These data depict an inverse relationship between leptin and various metabolic health outcomes in children. However, the notion that tOC or ucOC link fat with energy metabolism in healthy children was not supported.

Clinical trial registration number

NCT00931580.

Carboxylated and undercarboxylated osteocalcin in metabolic complications of human obesity and prediabetes

BACKGROUND

Carboxylated osteocalcin (Gla-OC) participates in bone remodeling, whereas the undercarboxylated form (Glu-OC) takes part in energy metabolism. This study was undertaken to compare the blood levels of Glu-OC and Gla-OC in nonobese, healthy obese, and prediabetic volunteers and correlate it with the metabolic markers of insulin resistance and early markers of inflammation.

METHODS

Nonobese (body mass index [BMI] <30 kg/m² ; n = 34) and obese subjects (30 <BMI <40 kg/m² ; n = 98), both sexes, aged 25 to 65 years, were divided into healthy control, normal weight subjects, healthy obese, and obese with biochemical markers of prediabetes. The subgroups with obesity and low or high Gla-OC or Glu-OC were also considered for statistical analysis. After 2 weeks of diet standardization, venous blood was sampled for the determination of Gla-OC, Glu-OC, lipid profile, parameters of inflammation (hsCRP, interleukin 6, sE-selectin, sPECAM-1, and monocyte chemoattractant protein 1), and adipokines (leptin, adiponectin, visfatin, and resistin).

RESULTS

Gla-OC in obese patients was significantly lower compared to nonobese ones (11.36 ± 0.39 vs 12.69 ± 0.90 ng/mL, P = .048) and weakly correlated with hsCRP (r = -0.18, P = .042), visfatin concentration (r = -0.19, P = .033), and BMI (r = -0.17, P = .047). Glu-OC was negatively associated with fasting insulin levels (r = -0.18, P = .049) and reduced in prediabetic individuals compared with healthy obese volunteers (3.04 ± 0.28 vs 4.48 ± 0.57, P = .025).

CONCLUSIONS

Decreased blood concentration of Glu-OC may be a selective early symptom of insulin resistance in obesity, whereas the decreased level of Gla-OC seems to be associated with the appearance of early markers of low grade inflammation accompanying obesity.

The rs2274911 polymorphism in GPRC6A gene is associated with insulin resistance in normal weight and obese subjects

OBJECTIVE

Identification of the novel endocrine role of osteocalcin (OC) and its receptor GPRC6A has given rise to a new branch of research in OC/GPRC6A axis related to glucose metabolism. GPRC6A- and OC-deficient mice share features of the metabolic syndrome, in addition to male infertility. Recently, the polymorphism rs2274911 in GPRC6A was shown to be associated with testicular impairment. We aimed to investigate the role of rs2274911 polymorphism in glucose and lipid metabolism in a cohort of normal weight and obese subjects DESIGN, PATIENTS, SETTINGS: A total of 392 male and females, including 218 obese patients and 174 age-matched normal weight controls, were retrospectively selected.

RESULTS

The distribution of rs2274911 alleles and genotypes did not differ either between normal weight and obese subjects or sexes (all P > 0·05). Age- and OC-adjusted multivariate analysis revealed that, in the normal weight group, fasting insulin and HOMA-IR increased in GA (P = 0·016 and P = 0·025) and AA genotypes (P = 0·033 and P = 0·040) compared with GG homozygotes. In the obese group, AA homozygotes had increased fasting glucose (P = 0·041 vs GG). Triglycerides, fasting insulin and HOMA-IR increased in both GA (P = 0·020, P < 0·001 and P = 0·001) and AA genotype (P = 0·021, P = 0·013 and P = 0·013).

CONCLUSION

In a cohort of normal weight and obese subjects, we found that the nonrare polymorphism rs2274911 in the GPRC6A gene was associated with insulin resistance features, independently of the metabolic phenotype and OC levels.

Osteoprotegerin is a Better Serum Biomarker of Coronary Artery Calcification than Osteocalcin in Type 2 Diabetes

OBJECTIVE

Coronary artery calcification (CAC) is a prominent feature of atherosclerosis and is associated with cardiovascular events. In vitro studies have suggested that osteoprotegerin (OPG) and osteocalcin (OC) exert anticalcification potential in the vessel wall. The objective of this study was to investigate the association of CAC and serum bone biomarkers in persons with type 2 diabetes.

METHODS

We examined 50 individuals with type 2 diabetes. CAC imaging was performed by multidetector computed tomography. CAC scores ≥10, expressed in Agatston units, were considered abnormal. OC, undercarboxylated OC (ucOC), and OPG levels were determined by enzyme-linked immunosorbent assay.

RESULTS

Abnormal CAC scores were found for 64% of the study cohort. OPG levels were significantly elevated (5.5 ± 2.0 pmol/L vs. 4.2 ± 1.7 pmol/L; P = .026) for those with abnormal CAC scores. No univariate differences were found for OC or ucOC. Logistic regression analyses revealed that an increase in serum OPG level was significantly associated with an increase in CAC score (odds ratio, 3.324; 95% confidence interval, 1.321 to 8.359; P = .011). Longer duration of diabetes was a significant covariate (P = .026), whereas nonsignificant covariates in the final model were age, gender, systolic blood pressure, body mass index, insulin resistance determined by the homeostasis model assessment for insulin resistance, leptin, adiponectin, and glycemic control. The Nagelkerke R2 for the model was 0.66. Neither OC nor ucOC were significantly associated with elevated CAC scores.

CONCLUSION

Our results suggest that OPG is a more useful serum biomarker than OC or ucOC for identifying those at increased risk of arterial calcification in type 2 diabetes.

Increased Gs Signaling in Osteoblasts Reduces Bone Marrow and Whole-Body Adiposity in Male Mice

Bone is increasingly recognized as an endocrine organ that can regulate systemic hormones and metabolism through secreted factors. Although bone loss and increased adiposity appear to be linked clinically, whether conditions of increased bone formation can also change systemic metabolism remains unclear. In this study, we examined how increased osteogenesis affects metabolism by using an engineered G protein-coupled receptor, Rs1, to activate Gs signaling in osteoblastic cells in ColI(2.3)(+)/Rs1(+) transgenic mice. We previously showed that these mice have dramatically increased bone formation resembling fibrous dysplasia of the bone. We found that total body fat was significantly reduced starting at 3 weeks of age. Furthermore, ColI(2.3)(+)/Rs1(+) mice showed reduced O2 consumption and respiratory quotient measures without effects on food intake and energy expenditure. The mice had significantly decreased serum triacylglycerides, leptin, and adiponectin. Resting glucose and insulin levels were unchanged; however, glucose and insulin tolerance tests revealed increased sensitivity to insulin. The mice showed resistance to fat accumulation from a high-fat diet. Furthermore, ColI(2.3)(+)/Rs1(+) mouse bones had dramatically reduced mature adipocyte differentiation, increased Wingless/Int-1 (Wnt) signaling, and higher osteoblastic glucose utilization than controls. These findings suggest that osteoblasts can influence both local and peripheral adiposity in conditions of increased bone formation and suggest a role for osteoblasts in the regulation of whole-body adiposity and metabolic homeostasis.

New insights into the biology of osteocalcin

Osteocalcin is among the most abundant proteins in bone and is produced exclusively by osteoblasts. Initially believed to be an inhibitor of bone mineralization, recent studies suggest a broader role for osteocalcin that extends to the regulation of whole body metabolism, reproduction, and cognition. Circulating undercarboxylated osteocalcin, which is regulated by insulin, acts in a feed-forward loop to increase β-cell proliferation as well as insulin production and secretion, while skeletal muscle and adipose tissue respond to osteocalcin by increasing their sensitivity to insulin. Osteocalcin also acts in the brain to increase neurotransmitter production and in the testes to stimulate testosterone production. At least one putative receptor for osteocalcin, Gprc6a, is expressed by adipose, skeletal muscle, and the Leydig cells of the testes and appears to mediate osteocalcin's effects in these tissues. In this review, we summarize these new discoveries, which suggest that the ability of osteocalcin to function both locally in bone and as a hormone depends on a novel post-translational mechanism that alters osteocalcin's affinity for the bone matrix and bioavailability. This article is part of a Special Issue entitled Bone and diabetes.

Vitamin D supplementation during short-term caloric restriction in healthy overweight/obese older women: Effect on glycemic indices and serum osteocalcin levels

The effect of vitamin D supplementation and caloric restriction (CR) on glycemic indices and osteocalcin (OC) is not clear. In this randomized controlled double blind trial, we examined whether vitamin D3 supplementation at 2500 IU/d (D) or placebo has differential effects on markers of insulin sensitivity and bone turnover in overweight/obese postmenopausal women during 6 weeks of caloric restriction (weight loss; WL, n = 39) compared to weight maintenance (WM, n = 37). Seventy-six women (57 ± 6 years) completed this study and the WL groups lost 4 ± 1% of body weight. Baseline serum 25-hydroxyvitamin D (25OHD) was 24.8 ± 5.6 ng/mL at baseline; the rise was greatest in WL-D group (p < 0.05). There was an interaction between vitamin D intake and weight on serum OC, insulin, glucose and markers of insulin sensitivity (p < 0.05). The change in OC was explained by changes in serum 25OHD and insulin (model R(2) = 25.6%). Overall, vitamin D supplementation and CR influence serum osteocalcin levels and modestly favor improvements in insulin sensitivity.

Childhood obesity, bone development, and cardiometabolic risk factors

Osteoporosis and obesity are both major public health concerns. It has long been considered that these are distinct disorders rarely found in the same individual; however, emerging evidence supports an important interaction between adipose tissue and the skeleton. Whereas overweight per se may augment bone strength, animal studies suggest that the metabolic impairment that accompanies obesity is detrimental to bone. Obesity during childhood, a critical time for bone development, likely has profound and lasting effects on bone strength and fracture risk. This notion has received little attention in children and results are mixed, with studies reporting that bone strength development is enhanced or impaired by obesity. Whether obesity is a risk factor for osteoporosis or childhood bone health, in general, remains an important clinical question. Here, we will focus on clarifying the controversial relationships between childhood obesity and bone strength development, and provide insights into potential mechanisms that may regulate the effect of excess adiposity on bone.

Effect of intermittent PTH treatment on plasma glucose in osteoporosis: A randomized trial

We investigated the effect of bone turnover on glucose homeostasis, fat distribution and adipokine production during anabolic treatment with PTH. This is a parallel, randomized controlled, open label, trial. The randomization was done by computer generated tables to allocate treatments. Forty-six postmenopausal osteoporotic non-diabetic women were assigned to treatment with calcium and colecalcipherol with (24) or without (22) PTH 1-84. Patients were recalled after 3, 6, 12 and 18 months of treatment and markers of bone turnover, glucose metabolism, adipokine secretion and fat distribution were analyzed. Markers of bone turnover and adipokines were measured by ELISA. Glucose metabolism was evaluated by an oral glucose load test and insulin resistance and secretion were calculated. Fat and lean mass were evaluated by anthropometric measures. The effect of treatment on measured variables was analyzed by repeated measure test, and its effect on glucose was also evaluated by mediation analysis after correction for possible confounders. Twenty patients in the calcium and vitamin D groups and 19 in the group treated with PTH 1-84 completed the study. There were no significance adverse events. Treatment with PTH increases osteocalcin, both total (OC) and undercarboxylated (uOC), and decreases blood glucose, without influence on insulin secretion, resistance and pancreatic β cell function. Treatment with PTH does not influence fat distribution and adipokine production. The results of the mediation analyses suggest a total effect of PTH on blood glucose, moderately mediated by OC and to a less extent by uOC. Here we suggest that treatment with PTH influences glucose metabolism partially through its effect on bone turnover, without influence on insulin secretion, resistance, pancreatic β cell function and fat mass.

Bone and the regulation of global energy balance

The skeleton, populated by large numbers of osteoblasts and long-lived osteocytes, requires a constant supply of energy-rich molecules to fuel the synthesis, deposition and mineralization of bone matrix during bone modelling and remodelling. When these energetic demands are not met, bone acquisition is suppressed. Recent findings suggest that key developmental signals emanating from Wnt low-density lipoprotein-related receptor 5 and hypoxia-inducible factor pathways impact osteoblast bioenergetics to accommodate the energy requirements for bone cells to fulfil their function. In vivo studies in several mutant mouse strains have confirmed a link between bone cells and global metabolism, ultimately leading to the identification of hormonal interactions between the skeleton and other tissues. The hormones insulin and leptin affect postnatal bone acquisition, whilst osteocalcin produced by the osteoblast in turn stimulates insulin secretion by the pancreas. These observations have prompted additional questions regarding the nature of the mechanisms of fuel sensing and processing in the osteoblast and their contribution to overall energy utilization and homeostasis. Answers to such questions should advance our understanding of metabolic diseases and may ultimately improve management of affected patients. In this review, we highlight recent studies in this field and offer a perspective on the evolutionary implications of bone as a metabolic endocrine organ.

The effects of aerobic exercise training on serum osteocalcin, adipocytokines and insulin resistance on obese young males

OBJECTIVE

Osteocalcin has been proposed to be a novel link between bone and energy metabolism. Previous studies showed its relations to exercise, body fat and glucose metabolism, but their interrelationship remains inconslusive. We evaluated the changes in osteocalcin level following 8-week exercise programme and assessed how they are related to concomitant changes in body fat composition, insulin resistance and various adipocytokines in a single centre, randomized and prospective design.

STUDY DESIGN AND METHODS

A total of 39 young obese, otherwise healthy males were randomly assigned to control (n = 10) and exercise (n = 29) groups. Subjects in Exercise group were on 8-week supervised exercise training programme of four sessions per week. Body fat compositions were analysed using whole body bone mineral density, various metabolic parameters, osteocalcin and adipocytokines were assessed from fasting blood samples before and after 8-week exercise programme.

RESULTS

Body fat reduction following exercise significantly increased serum total (1·51 ± 0·36 vs 1·69 ± 0·39 mmol/l, P = 0·01, baseline vs postexercise) and undercarboxylated osteocalcin level (0·44 ± 0·14 vs 0·64 ± 0·26 mmol/l, P < 0·01), and the increase in osteocalcin was in negative correlations with changes in body weight, BMI and body fat percentage as well as HOMA-IR and leptin (all P < 0·05). The changes in osteocalcin and leptin were not independent predictors of changes in insulin resistance and osteocalcin, respectively.

CONCLUSIONS

In a physiological axis of bone-fat-energy metabolism, exercise-induced body fat reduction and improved insulin sensitivity were accompanied by an increase in serum osteocalcin and leptin levels, but other factors also seem to be involved in this interrelationship.

An update on diabetes related skeletal fragility

There are several mechanisms by which diabetes could affect bone mass and strength. These mechanisms include insulin deficiency; hyperglycemia; the accumulation of advanced glycation end products that may influence collagen characteristics; marrow adiposity and bone inflammation. Furthermore, associated diabetic complications and treatment with thaizolidinediones may also increase risk of fracturing. The following article provides its readers with an update on the latest information pertaining to diabetes related bone skeletal fragility. In the authors' opinion, future studies are needed in order to clarify the impact of different aspects of diabetes metabolism, glycemic control, and specific treatments for diabetes on bone. Given that dual energy x-ray absorptiometry is a poor predictor of bone morbidity in this group of patients, there is a need to explore novel approaches for assessing bone quality. It is important that we develop a better understanding of how diabetes affects bone in order to improve our ability to protect bone health and prevent fractures in the growing population of adults with diabetes.

Osteocalcin protects against nonalcoholic steatohepatitis in a mouse model of metabolic syndrome

Nonalcoholic fatty liver disease, particularly its more aggressive form, nonalcoholic steatohepatitis (NASH), is associated with hepatic insulin resistance. Osteocalcin, a protein secreted by osteoblast cells in bone, has recently emerged as an important metabolic regulator with insulin-sensitizing properties. In humans, osteocalcin levels are inversely associated with liver disease. We thus hypothesized that osteocalcin may attenuate NASH and examined the effects of osteocalcin treatment in middle-aged (12-mo-old) male Ldlr(-/-) mice, which were fed a Western-style high-fat, high-cholesterol diet for 12 weeks to induce metabolic syndrome and NASH. Mice were treated with osteocalcin (4.5 ng/h) or vehicle for the diet duration. Osteocalcin treatment not only protected against Western-style high-fat, high-cholesterol diet-induced insulin resistance but substantially reduced multiple NASH components, including steatosis, ballooning degeneration, and fibrosis, with an overall reduction in nonalcoholic fatty liver disease activity scores. Further, osteocalcin robustly reduced expression of proinflammatory and profibrotic genes (Cd68, Mcp1, Spp1, and Col1a2) in liver and suppressed inflammatory gene expression in white adipose tissue. In conclusion, these results suggest osteocalcin inhibits NASH development by targeting inflammatory and fibrotic processes.

Bone as an endocrine organ relevant to diabetes

There are well-established associations between diabetes and fracture risk and yet the mechanism underlying these associations are controversial. Guided by a series of mouse studies, a specific form of the bone protein, osteocalcin, was proposed to be the mechanistic link between these two chronic diseases. Translation to humans initially appeared elusive in part because serum concentrations of osteocalcin are a biomarker of bone turnover and not necessarily specific to the biology of this protein. The suitability of the mouse model for the study of osteocalcin as a therapeutic target also appears ambiguous. With greater discrimination of the different forms of osteocalcin present in circulation and inclusion of multiple measures of bone turnover, evidence currently does not support osteocalcin as a protein critical to the diabetes and fracture association in humans.

Vitamin K: dietary intake and requirements in different clinical conditions

PURPOSE OF REVIEW

Vitamin K is an enzyme cofactor for the carboxylation of vitamin K-dependent proteins. Functions include coagulation and regulation of calcification. Different clinical conditions may alter vitamin K requirements by affecting vitamin K status and vitamin K-dependent proteins carboxylation that are reviewed here.

RECENT FINDINGS

Vitamin K consumption greater than the current usual daily requirement to maintain health is indicated for prevention of vitamin K-deficient bleeding in infants and for rescue of over-anticoagulation in patients on vitamin K-dependent oral anticoagulants. Additional vitamin K intake may be required in malabsorptive conditions such as cystic fibrosis and following bariatric surgery. Carboxylation of vitamin K-dependent proteins occurs in multiple extrahepatic tissues and has been implicated in soft tissue calcification and insulin resistance, although the exact mechanisms have yet to be determined. Contribution of colonic flora to vitamin K requirements remains controversial.

SUMMARY

With the increased incidence of vitamin K-deficient bleeding and weight-loss surgical procedures, healthcare professionals need to monitor vitamin K status in certain patient populations. Future research on the roles of vitamin K in extrahepatic tissues as they pertain to chronic disease will provide insight into the therapeutic potential of vitamin K and lead to the development of recommendations for specific clinical populations.

γ-Glutamyl carboxylase in osteoblasts regulates glucose metabolism in mice

Vitamin K-dependent γ-glutamyl carboxylase (GGCX) is an enzyme that catalyzes the conversion of glutamic acid to gamma-carboxyglutamic acid in substrate proteins. Among GGCX target proteins, recent evidence indicates that osteocalcin regulates insulin sensitivity and secretion. However, the precise contribution of GGCX to glucose metabolism remains to be clarified. To address this question, we generated osteoblast-specific Ggcx-deficient (i.e., conditional knockout [cKO]) mice using collagen type 1 α1 (Col1)-Cre mice. Ggcx cKO mice exhibited altered metabolism compared with their controls; serum glucose levels could be maintained with low amounts of insulin, and the weight of white adipose tissue (WAT) significantly decreased in Ggcx cKO mice. Our findings suggest that GGCX expressed in osteoblasts is critical for the maintenance of blood glucose and WAT.

Increasing adiposity is associated with higher adipokine levels and lower bone mineral density in obese older adults

CONTEXT

Although obesity is associated with high bone mass, recent reports suggest an increase in the incidence of fractures in obese patients.

OBJECTIVES

The objectives of the study were to evaluate the influence of increasing body fat on bone mineral density (BMD) and to determine the influence of the different adipokines on BMD in frail obese elderly patients.

DESIGN AND SETTING

This is a cross-sectional study of baseline characteristics of elderly obese patients participating in a lifestyle therapy with diet with or without exercise and conducted in a university setting.

PATIENTS

One hundred seventy-three, elderly (≥65 y old), obese (body mass index of ≥30 kg/m(2)) who were mostly frail participated in the study.

OUTCOME MEASURES

BMD, percentage of total body fat, percentage of fat-free mass, percentage of lean mass, body mass index, adiponectin, leptin, IL-6, bone turnover markers (osteocalcin and C-telopeptide), high-sensitivity C-reactive protein, free estradiol, and 25-hydroxyvitamin D were measured.

RESULTS

Higher tertiles of percentage body fat and lower lean mass were associated with a lower BMD. High-sensitivity C-reactive protein levels were highest in the highest fat tertile (third, 5.5 ± 5.4 vs first, 1.5 ± 1.3 mg/L, P < .05) for women, whereas IL-6 levels were highest in the highest tertile in men (third, 3.5 ± 3.1 vs first, 1.7 ± 0.8 pg/mL, P < .05). Leptin increased with increasing fat tertiles in both genders (P < .05), whereas adiponectin increased with increasing fat tertiles only in men (P < .05). A multivariate analysis revealed adiponectin as an important mediator of the effect of fat mass on BMD. Osteocalcin levels were highest in the highest fat tertile in women but not in men. Physical function test scores decreased with increasing fat tertiles in women (P < .05) but not in men.

CONCLUSIONS

Increasing adiposity together with decreasing lean mass is associated with lower BMD, higher adipokine levels, and worsening frailty in elderly obese adults.

Insulin and bone: Recent developments

While insulin-like growth factor I is a well-known anabolic agent in bone evidence is beginning to accumulate that its homologue, insulin, also has some anabolic properties for bone. There is specific evidence that insulin may work to stimulate osteoblast differentiation, which in turn would enhance production of osteocalcin, the osteoblast-produced peptide that can stimulate pancreatic β cell proliferation and skeletal muscle insulin sensitivity. It is uncertain whether insulin stimulates bone directly or indirectly by increasing muscle work and therefore skeletal loading. We raise the question of the sequence of events that occurs with insulin resistance, such as type 2 diabetes. Evidence to date suggests that these patients have lower serum concentrations of osteocalcin, perhaps reduced skeletal loading, and reduced bone strength as evidenced by micro-indentation studies.