Diabetes Metab Res Rev. 2017 Mar;33(3).

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

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Razny U, Fedak D, Kiec-Wilk B, Goralska J, Gruca A, Zdzienicka A, Kiec-Klimczak M, Solnica B, Hubalewska-Dydejczyk A, Malczewska-Malec M (2017) Carboxylated and undercarboxylated osteocalcin in metabolic complications of human obesity and prediabetes. Diabetes Metab Res Rev 33(3). doi: 10.1002/dmrr.2862.

 

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Osteoporosis and obesity are two global health problems. Body fat seem to interfere with bone health through adipose tissue derived mediators (adipokines) and may increase a risk for osteoporosis and fragility fractures. In turn bone derived molecules could modulate not only bone remodelling but also adipogenesis and energy metabolism. There is a cross talk between adipose and bone tissue through several mechanisms. Osteocalcin (OC) is a protein specially synthesized and secreted by cells of osteoblast lineage (osteoblasts and osteocytes) and seem to participate in a cross talk between adipose and bone tissue. It is an osteoblast specific, noncollagen protein, component of bone extracellular matrix which is secreted to stimulate osteoblastic differentiation and osteocytic maturation. The mechanism of osteocalcin action is dependent on carboxylation of three glutamyl residues. In a posttranslational modification, carboxylation of the three glutamyl residues (Glu) by a vitamin-K-dependent γ-glutamyl carboxylase, leads to a conformation change, what stabilizes the α-helical structure of the protein. Carboxylated osteocalcin (Gla-OC) could bind Ca2+ and hydroxyapatite and is deposited into the bone extracellular matrix. However, 15% of new synthetised Gla-OC is released into circulation and is considered as a marker of bone formation and bone remodelling. In blood circulation main form of the protein is undercarboxylated osteocalcin (Glu-OC) which does not bind Ca2+ and hydroxyapatite, but is known as biologically active molecule participating in glucose and lipid metabolism. Plasma level of Glu-OC is regulated by decarboxylation of osteocalcin and its releasing from the bone matrix during bone resorption due to acidic environment which facilitates decarboxylation of osteocalcin. Our findings [1] confirmed that osteocalcin is an osteoblast-specific secreted protein, which participates not only in bone remodeling but also in glucose metabolism regulation. Results of our study also indicate that proinflammatory cytokines secreted by adipose tissue influence osteocalcin level. In our studies we have compared the level of carboxylated (Gla-OC) and undercarboxylated (Glu-OC) osteocalcin in blood circulation in non-obese and obese subjects. Participants of the study were potentially healthy without diseases that could affect the metabolism of glucose and lipids (diabetes mellitus, pregnancy, endocrine disorders, kidney or liver dysfunction and other chronic diseases). They did not take any medication except for hipotensive drugs (metabolically neutral). Obese subjects who presented higher fasting insulin level, increased insulin resistance index HOMA-IR, higher plasma concentration of leptin as well as proinflammatory cytokines (IL-6, hsCRP), had decreased plasma Gla-OC level in comparison to non-obese ones (Figure 1). In addition, we have found that subjects with the plasma value of Gla-OC higher than median value had lower plasma concentration of proinflammatory cytokines secreted by adipose tissue: visfatin and hsCRP in comparison to the group of subjects presenting lower than median value of Gla-OC (Figure 2). In turn, comparison of obese healthy subjects (without any metabolic disturbances) with obese volunteers presented prediabetic state (with or without atherogenic dyslipidemia) revealed statistically significant decrease in plasma Glu-OC level in subjects with prediabetes than control ones (Figure 3). Moreover, the obese subjects with level of Glu-OC higher than median had statistically significant lower fasting insulin and HOMA-IR index in comparison to subjects with lower than median value of Glu-OC (Figure 4). Thus, lower plasma level of Glu-OC is associated and could be an early symptom of insulin resistance in obesity. As regards association of osteocalcin and proinflammatory cytokines, we are the first to show an inverse association of carboxylated osteocalcin with visfatin and hsCRP in humans. However, inflammation in obesity could be linked with increased oxidative stress. Obese people with disturbances in glucose or/and insulin resistance have also in blood as well as urine increased level of advanced glycation end products (AGEs) which are formed by non-enzymatic reaction of proteins with saccharides and related metabolites. Our obese volunteers in comparison to non-obese ones presented higher urine level of early stage glycation adduct Nε-fructosyl-lysine (FL) residues formed by reaction of glucose with proteins [2]. In urine of obese participants also elevated level of hydroimidazolone Nδ-(5-hydro-5-(2.3.4-trihydroxybutyl)-4-imidazolon-2-yl)ornithine (3DG-H) residues was found, which is the major AGE formed by direct reaction of reactive dicarbonyl metabolite 3-deoxyglucosone (3-DG) with proteins. In a group of obese subjects we have also found increased level of protein oxidation product – N-formylkynurenine (NFK) residues which is a major product of oxidative damage to Tryptophan. Moreover, NFK free adduct was inversely associated with plasma Gla-OC level [2]. It could be suggested that lower level of Gla-OC in obese subjects could be a marker of low grade inflammation in obesity. Gla-OC could be affected by inflammation as well as oxidative stress in obesity, what in future could lead to disturbances in bone formation.

 

 

Figure 1. Blood Glu-OC and Gla-OC level in non-obese (n=34) and whole group of obese subjects (n=98). *P<0.05 [data adapted from Razny et al., Diab. Met. Res. Rev 2017]

 

 

Figure 2. Comparisons of two subgroups with a higher (n=66) and lower (n=66) than median value of carboxylated osteocalcin (Gla-OC) level selected from the obese group of participating subjects. The calculated median was considered. *P<0.05 [data adapted from Razny et al., Diab. Met. Res. Rev 2017]

 

 

Figure 3. Blood Glu-OC and Gla-OC level in obese healthy (n=29) and obese with prediabetes subjects (n=32). *P<0.05 [data adapted from Razny et al., Diab. Met. Res. Rev 2017]

 

 

Figure 4. Comparisons of two subgroups obese subjects with a higher (n=49) and lower (n=49) than median value of undercarboxylated osteocalcin (Glu-OC) level selected from the obese group of participating subjects. The calculated median was considered. *P<0.05 [data adapted from Razny et al., Diab. Met. Res. Rev 2017 ]

 

 

References:

  1. Razny U, Fedak D, Kiec-Wilk B, Goralska J, Gruca A, Zdzienicka A, Kiec-Klimczak M, Solnica B, Hubalewska-Dydejczyk A, Malczewska-Malec M (2017) Carboxylated and undercarboxylated osteocalcin in metabolic complications of human obesity and prediabetes. Diabetes Metab Res Rev 33(3).
  2. Razny U, Goralska J, Anna Zdzienicka A, Danuta Fedak D, Masania J, Rabbani N, Thornalley P, Pawlica-Gosiewska D, Gawlik K, Dembinska-Kiec A, Solnica B, Malgorzata Malczewska-Malec M Relation of protein glycation, oxidation and nitration with osteocalcin level in obese subjects. Accepted for publication in Acta Biochimica Polonica (June 19th 2017)