The roles of osteocalcin in lipid metabolism in adipose tissue and liver.

Bone provides skeletal support and functions as an endocrine organ by producing osteocalcin, whose uncarboxylated form (GluOC) increases the metabolism of glucose and lipid by activating its putative G protein-coupled receptor (family C group 6 subtype A). Low doses (≤10 ng/ml) of GluOC induce the expression of adiponectin, adipose triglyceride lipase and peroxisome proliferator-activated receptor γ, and promote active phosphorylation of lipolytic enzymes such as perilipin and hormone-sensitive lipase via the cAMP-PKA-Src-Rap1-ERK-CREB signaling axis in 3T3-L1 adipocytes. Administration of high-dose (≥20 ng/ml) GluOC induces programmed necrosis (necroptosis) through a juxtacrine mechanism triggered by the binding of Fas ligand, whose expression is induced by forkhead box O1, to Fas that is expressed in adjacent adipocytes. Furthermore, expression of adiponectin and adipose triglyceride lipase in adipocytes is triggered in the same manner as following low-dose GluOC stimulation; these effects protect mice from diet-induced accumulation of triglycerides in hepatocytes and consequent liver injury through the upregulation of nuclear translocation of nuclear factor-E2-related factor-2, expression of antioxidant enzymes, and inhibition of the c-Jun N-terminal kinase pathway. Evaluation of these molecular mechanisms leads us to consider that GluOC might have potential as a treatment for lipid metabolism disorders. Indeed, there have been many reports demonstrating the negative correlation between serum osteocalcin levels and obesity or non-alcoholic fatty liver disease, a common risk factor for which is dyslipidemia in humans. The present review summarizes the effects of GluOC on lipid metabolism as well as its possible therapeutic application for metabolic diseases including obesity and dyslipidemia.

Regulation of Atrial Fibrosis by the Bone.

MR (mineralocorticoid receptor) antagonists have been demonstrated to provide beneficial effects on preventing atrial fibrosis. However, the underlying cellular and molecular mechanisms remain unclear. We aim to determine the role of osteoblast MR in atrial fibrosis and to explore the underlying mechanism. Using osteoblast MR knockout mouse in combination with mutant TGF (transforming growth factor)-ß1 transgenic mouse, we demonstrated that MR deficiency in osteoblasts significantly attenuated atrial fibrosis. Mechanistically, MR directly regulated expression of OCN (osteocalcin) in osteoblasts. Both carboxylated and undercarboxylated OCNs (ucOC) were less secreted in osteoblast MR knockout mice. Mutant TGF-ß1 transgenic mice supplemented with recombinant ucOC showed aggravated atrial fibrosis. In cultured atrial fibroblasts, ucOC treatment promoted proliferation and migration of atrial fibroblasts, whereas cotreatment with an antagonist for a GPRC6A (G-protein-coupled receptor, family C, group 6, member A) abolished these effects. Western blotting analysis revealed upregulation of PKA (protein kinase A) and CREB (cAMP-response element-binding protein) phosphorylation after ucOC treatment. Inhibition of PKA with its antagonist reduced ucOC-induced proliferation and migration of atrial fibroblasts. Finally, the impact of osteoblast MR deficiency on atrial fibrosis was abolished by ucOC administration in mutant TGF-ß1 transgenic mice. Taken together, MR deficiency in osteoblasts attenuated atrial fibrosis by downregulation of OCN to promote proliferation and migration of atrial fibroblasts.

Vitamin K-induced effects on body fat and weight: results from a 3-year vitamin K2 intervention study.

BACKGROUND/OBJECTIVES: Vitamin K status has been linked to fat and glucose metabolism by several authors, but whether high vitamin K intake influences body weight or composition has remained unclear. Here we tested the hypothesis that increased vitamin K intake decreases body fat or fat distribution. SUBJECTS/METHODS: In a randomized placebo-controlled human intervention trial, 214 postmenopausal women, 55-65 years of age, received either 180 mcg/day of vitamin K2 (menaquinone-7, MK-7) or placebo for 3 years. Osteocalcin (OC) carboxylation was used as a marker for vitamin K status, and fat distribution was assessed by dual-energy X-ray absorptiometry total body scan. RESULTS: In the total cohort, MK-7 supplementation increased circulating carboxylated OC (cOC) but had no effect on body composition. In those with an above-median response in OC carboxylation ('good responders'), MK-7 treatment resulted in a significant increase in total and human molecular weight adiponectin and a decrease in abdominal fat mass and in the estimated visceral adipose tissue area compared with the placebo group and the poor responders. CONCLUSIONS: The fact that changes in body composition measures or markers for fat or glucose metabolism were not associated with changes in uncarboxylated OC (ucOC) does not support the assumption that ucOC stimulates fat metabolism in humans. Instead, high vitamin K2 intake may support reducing body weight, abdominal and visceral fat, notably in subjects showing a strong increase in cOC. A causal relation between the changes in cOC and body fat or distribution cannot be concluded from these data.

Has sclerostin a true endocrine metabolic action complementary to osteocalcin in older men?

UNLABELLED: The reported association between sclerostin and diabetes mellitus or abdominal fat may be biased by body size and bone mass. In older men, the association between serum sclerostin levels and metabolic syndrome lost significance after adjustment for bone mass. The association between sclerostin and energy metabolism needs further clarification. INTRODUCTION: Sclerostin is associated with abdominal fat, but this relationship may be biased since both are associated with body size and bone mass. Osteocalcin is a bone-derived hormone regulating energy metabolism. We assessed the association between serum sclerostin and metabolic syndrome (MetS) accounting for whole body mineral content (BMC) and osteocalcin. METHODS: We studied 694 men aged 51-85 who had serum osteocalcin and sclerostin measurements. RESULTS: Sclerostin was higher in 216 men with MetS compared with those without MetS (p < 0.005). Average sclerostin level increased significantly across the increasing number of MetS components. In multivariable models, higher sclerostin was associated with higher odds of MetS (odds ratio (OR) = 1.24/1 standard deviation (SD) increase [95 % confidence interval (95 % CI), 1.01-1.51]; p < 0.05). After further adjustment for BMC, the association of MetS with sclerostin lost significance, whereas that with osteocalcin remained significant. Men who were simultaneously in the highest sclerostin quartile and the lowest osteocalcin quartile had higher odds of MetS (OR = 2.14 [95 % CI, 1.15-4.18]; p < 0.05) vs. men being in the three lower sclerostin quartiles and three upper osteocalcin quartiles. After adjustment for whole body BMC, the association lost significance. CONCLUSIONS: Higher sclerostin level is associated with MetS severity; however, this association may be related to higher whole body BMC. The adjustment for BMC had no impact on the association between MetS and osteocalcin. Clinical cross-sectional studies do not elucidate the potential role of sclerostin in the regulation of energy metabolism and direct experimental approach is necessary.

Acción de la vitamina K sobre la salud ósea / Action of vitamin K on bone health

Objetivos: Se admite hoy en día que la vitamina K tiene un papel importante en la salud ósea. Es necesaria para la gamma-carboxilación de la osteocalcina (la proteína no colágena más importante en el hueso), consiguiendo que la osteocalcina funcione. Hay dos formas importantes de la vitamina K (vitamina K1 y vitamina K2), que provienen de diferentes fuentes y tienen diferentes actividades biológicas. Estudios epidemiológicos sugieren que una dieta con niveles altos de vitamina K se asocia con un menor riesgo de fracturas de cadera en hombres ancianos y mujeres. Sin embargo, ensayos clínicos controlados y aleatorizados, realizados con suplementos de vitamina K1 o K2 en la población blanca, no muestran un aumento en la densidad mineral ósea (DMO) en la mayoría de las diferentes partes del esqueleto. Los suplementos con vitamina K1 y K2 pueden reducir el riesgo de fractura, pero los ensayos clínicos que incluyen las fracturas como resultado final tienen limitaciones metodológicas, por lo que se necesitarían ensayos clínicos con mayor número de pacientes y mejor diseñados para comprobar la eficacia de la vitamina K1 y K2 en las fracturas. Como conclusión, podríamos decir que actualmente no existe una evidencia suficiente para recomendar el uso rutinario de suplementos de vitamina K para la prevención de la osteoporosis y las fracturas en mujeres postmenopáusicas (AU)

Objetives: Nowadays it is recognised that vitamin K plays an important role in bone health. It is necessary for the gamma-carboxylation of osteocalcin (the most important non-collagen protein in the bone), making the osteocalcin function. There are two important forms of vitamin K (vitamin K1 and vitamin K2), which come from different sources and have different biological activity. Epidemiological studies suggest that a diet with high levels of vitamin K is associated with a lower risk of hip fractures in older men and in women. However, controlled randomised clinical trials, carried out with supplements of vitamin K1 or K2 in the white population do not show an increase in bone mineral density (BMD) in most of the different areas of the skeleton. Supplementation with vitamin K1 and K2 may reduce the risk of fracture, but the clinical trials which include fractures as a final result have methodological limitations, so clinical trials with greater numbers of patients, and which are better designed, would be needed in order to prove the efficacy of vitamin K1 and K2 in relation to fractures. In conclusion, we may say that there is currently insufficient evidence to recommend the routine use of vitamin K for the prevention of osteoporosis and fractures in postmenopausal women (AU)

Biology without walls: the novel endocrinology of bone.

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

Novel nonsecosteroidal vitamin D3 carboxylic acid analogs for osteoporosis, and SAR analysis.

Novel vitamin D(3) analogs with carboxylic acid were explored, focusing on a nonsecosteroidal analog, LG190178, with a bisphenyl skeleton. From X-ray analysis of these analogs with vitamin D receptor (VDR), the carboxyl groups had very unique hydrogen bonding interactions in VDR and mimicked 1α-hydroxy group and/or 3ß-hydroxy group of 1α,25-dihydroxyvitamin D(3). A highly potent analog, 6a, with good in vitro activity and pharmacokinetic profiles was identified from an SAR study. Compound 6a showed significant prevention of bone loss in a rat osteoporosis model by oral administration.

[Control of calcification by extracellular matrix].

Physiological calcification begins from crystallization of hydroxyapatite in extracellular matrix of both bones and teeth. Because calcification is exactly the extracellular event, organic components of extracellular matrix play important roles in control of calcification. The authors discuss the molecular regulation of calcification by organic components of extracellular matrix, focusing on the mineral/organic interaction.

Vitamin D control of osteoblast function and bone extracellular matrix mineralization.

Vitamin D is the major regulator of calcium homeostasis and protects the organism from calcium deficiency via effects on the intestine, kidney, parathyroid gland, and bone. Disturbances in the vitamin D endocrine system (e.g., vitamin D-dependent rickets type I and type II), result in profound effects on the mineralization of bone. Recent studies with vitamin D receptor knockout mice also show effects on bone. It is questioned whether vitamin D has a direct effect on bone formation and mineralization. In rickets and particular vitamin D receptor knockout mice, calcium supplementation restores bone mineralization. However, the vitamin D receptor is present in osteoblasts, and vitamin D affects the expression of various genes in osteoblasts. This review focuses on the role of vitamin D in the control of osteoblast function and discusses the current knowledge of the direct effects of vitamin D on mineralization. Moreover, the role of vitamin D metabolism and the mechanism of action of vitamin D and interaction with other hormones and factors are discussed.

Zinc and perinatal growth.

AIM: To study the relationship between zinc, insulin-like growth factor (IGF-I), osteocalcin and perinatal growth. MATERIALS AND METHODS: Anthropometric variables, serum levels of IGF-I, osteocalcin and zinc were measured in preterm (PT) appropriate for gestational age (AGA), full term (FT) small for gestational age (SGA) and FT AGA newborns at birth (n=52), at first week (n=38) and at third week (n=38) of postnatal age. RESULTS: At birth, the FT SGA had lower levels of IGF-I. At the first week after birth, both FT SGA and PT AGA infants had less deposits of subcutaneous fat; FT SGA infants also showed significantly lower osteocalcin and zinc levels. At 3 weeks of postnatal life, FT SGA infants continued to show significantly lower zinc levels. They had increased their fat deposits so that differences between groups in skinfold measurements failed to be significant. The highest levels of osteocalcin were found in PT AGA infants at the third week after birth. CONCLUSIONS: Perinatal nutrition influences the IGF-I levels. The low concentration of osteocalcin found in SGA infants at first week of life probably reflects decreased bone turnover. During the first weeks of life, there is a progressive decrease in the serum levels of zinc, accentuated in the FT SGA group.

The determination of serum concentrations of osteocalcin in growing pigs and its relationship to end-measures of bone mineralization.

Osteocalcin, a 49-amino acid, gamma-carboxyglutamic acid-containing protein produced by the osteoblast, has been shown in laboratory animals to be a better marker of bone turnover than alkaline phosphatase. To determine serum osteocalcin levels in growing pigs, we isolated pure porcine osteocalcin and developed a double-antibody RIA. To evaluate the effects of dietary Ca and P levels on serum osteocalcin, 36 individually penned crossbred pigs (19.5 kg initial BW) were fed fortified corn-soybean meal diets (.95% lysine) containing four levels of Ca (.42, .66, .90, 1.14%) and P (.35, .55, .75, .95%) in a 30-d test. Increasing dietary Ca and P improved body weight gain quadratically (P < .02). Most bone traits improved quadratically (P < .05) with increasing Ca and P. Pigs were bled on d 0, 10, 20, and 30 to determine serum levels of alkaline phosphatase, 1,25-dihydroxyvitamin D3, and osteocalcin. Osteocalcin decreased (P < .02) linearly with increasing Ca and P on d 10, 20, and 30. However, this effect was much more pronounced on d 20 and 30. Alkaline phosphatase decreased with the first incremental increase in dietary Ca and P, but was not affected by higher levels on any day measured. Osteocalcin was inversely correlated with growth rate (r = -.54, P < .01), bone strength (r = -.57, P < .01), metacarpal ash (r = -.29, P < .10), femur ash (r = -.60, P < .01), and femur ash weight (r = -.65, P < .01). Similar results were found for 1,25-dihydroxyvitamin D3. Alkaline phosphatase was not correlated with performance or most bone traits on d 30. Based on this model, these results suggest that serum osteocalcin and 1,25-dihydroxyvitamin D3 are better predictors of bone mineralization and(or) turnover in pigs than serum alkaline phosphatase.