Introduction
Osteocalcin is a synthetic osteoblast-specific protein which is involved in the regulation of bone, glucolipid metabolism and in maintaining homeostasis [
1‐
6]. Recent studies have investigated whether osteocalcin is associated with subclinical and clinical atherosclerotic coronary artery disease (CAD), but these studies have yielded inconsistent results. In some cases, studies have found inverse associations between osteocalcin and CAD [
7‐
12]. For example, several studies have shown that subjects with CAD had significantly lower serum osteocalcin level than those without CAD [
8,
9]. Similarly, serum osteocalcin level in subjects with type 2 diabetes mellitus (T2DM) has been found to be inversely correlated with subclinical measures of CAD, such as carotid intima-media thickness (C-IMT) and plaque score, independent of confounding factors [
10‐
12].
In contrast to studies reporting an inverse association, some studies have reported no association or a positive association between serum osteocalcin level and measures of CAD. For example, population-based longitudinal study by Holvik et al. found that there was no association between plasma osteocalcin level and cardiovascular diseases (CVD) in younger-old subjects (65–74 years), but a higher plasma osteocalcin concentration was associated with a reduced risk of CVD in older-old men (≥75 years) while with an increased risk of CVD in older-old women (≥75 years) [
13]. However, other research by Reyes-Garcia et al. observed that subjects with CAD had significantly higher serum osteocalcin level than those without CAD, and that serum osteocalcin level in women was positively correlated with C-IMT [
14].
It is unclear why the studies noted above have reported contradictory findings on the association between serum osteocalcin level and atherosclerosis. The lack of consistency might be due to different study populations, or different degrees of confounding from factors that are associated with serum osteocalcin level, such as metabolic-related variables. In any event, the inconsistent results suggest that additional research into the association between osteocalcin and atherosclerosis is warranted, particularly in metabolically health populations that could help limit confounding from metabolic factors. Accordingly, the present study investigated the association between serum osteocalcin level and subclinical atherosclerosis in metabolically normal subjects without a history of CVD.
Discussion
Evidences indicate that skeleton works not only as a structural scaffold but also as an endocrine organ. Many bioactive factors secreted from bone, such as osteocalcin and osteoprotegerin, have a regulatory role in energy metabolism [
21,
22]. Recent studies have suggested a potentially important role for osteocalcin in regulating the function of vascular endothelial cells, and the cardiovascular system [
23,
24]. This suggests osteocalcin could be involved in the development of CAD. Several studies have investigated the association between osteocalcin and atherosclerosis, but results have not been consistent. Some studies suggest osteocalcin may actually be protective against early atherosclerosis. For example, a study from Korea demonstrated an independent effect of osteocalcin on vascular endothelial cells, suggesting that osteocalcin could have beneficial effects on atherosclerosis [
25]. Our previous study in high fat diet animals indicate that osteocalcin has an endothelial-protective effect in atherosclerosis through mediating the PI3K/Akt/eNOS signaling pathway, and exogenous osteocalcin can improve the function of human umbilical vein endothelial cells
in vitro [
26]. Consistent with this result, a study in 1319 postmenopausal women found serum osteocalcin level was independently and inversely correlated with C-IMT [
27].
The arteries plaques and aortic calcifications would emerge in the development and progression of atherosclerosis. Moreover, coronary atherosclerosis is the major adverse cardiac events of atherosclerosis. Likewise, a study from Japan of 50 patients with T2DM found that changes in the serum osteocalcin level were significantly and inversely correlated with plaque score [
12]. Another study of 774 elderly men found that higher osteocalcin level was associated with lower abdominal aortic calcification progression [
28]. Additionally, our group and others have found inverse associations between serum osteocalcin level and coronary atherosclerosis in Chinese men [
8,
9].
However, in contrast to the studies above, some data indicate that osteocalcin shows no association, or even shows a positive association, with atherosclerosis or CVD. Thus, a follow-up study of 1290 Korean men found that the serum osteocalcin level was not associated with the development of CVD, even after adjustment for other risk factors for CVD [
29]. Another 4.1-year follow-up study also reported no association between plasma osteocalcin level and CVD in younger-old subjects (65–74 years) [
13]. A positive correlation between serum osteocalcin level and atherosclerosis was found in a study of 78 patients with T2DM. In particular, the level of osteocalcin in subjects with CAD was higher than in those without CAD, and higher concentrations of osteocalcin were found in women with abnormal IMT, carotid plaques, and aortic calcifications compared to normal women [
14].
It is unclear why the results noted above have been so inconsistent. One possible explanation is that the studies were conducted in different populations and ethnic groups. Another explanation may be that different studies were affected to different degrees by confounding from metabolic factors. Studies have demonstrated there is a close correlations between serum osteocalcin level and glucolipid metabolic disorders, obesity, and metabolic syndrome. Each of these metabolic dysfunctions are related to the progression of atherosclerosis, and therefore, are considered as risk factors for CVD [
30]. Researches indicate that subjects with diabetes or impaired glucose tolerance have significantly lower serum osteocalcin level than the subjects with normal glucose tolerance [
31,
32], and that serum osteocalcin level of subjects with metabolic syndrome are lower than in those without metabolic syndrome [
33]. These findings suggest that the association of osteocalcin with atherosclerosis may be influenced by metabolic variables. Thus, to eliminate potential confounding from metabolic variables, it might be useful to explore the relationship between serum osteocalcin level and atherosclerosis in metabolically healthy persons. In this regard, a study of 638 men with normal glucose tolerance by Ma et al. showed an inverse association between serum osteocalcin level and the prevalence of carotid plaque. However, the study did not account for the influence of other metabolic factors, such as obesity, hypertension and dyslipidemia [
31]. Therefore, to better clarify the relationship of serum osteocalcin level to atherosclerosis without confounding from metabolic variables, the present study excluded subjects with traditional risk factors for CVD (overweight, obesity, hyperglycemia, hypertension, and dyslipidemia), and further eliminated those with history of CVD or carotid plaque. In accordance with the previous findings of gender-related difference in serum osteocalcin concentration [
32], the serum osteocalcin level of postmenopausal women was significantly higher than those of men and premenopausal women in this study. There was no statistically significant correlation between serum osteocalcin level and C-IMT in either men or postmenopausal women. Although an simple correlation between these two factors was found in premenopausal women, a multivariable model failed to demonstrate a significant association between serum osteocalcin level and C-IMT after adjusting for relevant confounding factors. Our previous study involving animal models also found that exogenous osteocalcin was not related to endothelium-dependent relaxation in mice fed with chow diet, which is in agreement with the present study’s finding of metabolically healthy human to some extent [
26].
Limitations
Because of the cross-sectional nature of this study, we were not able to clarify whether the association between osteocalcin and C-IMT was causal. Thus, additional prospective studies are needed to assess this association. In addition, serum total osteocalcin is composed of uncarboxylated and carboxylated forms. Initial animal and
in vitro studies indicated that undercarboxylated osteocalcin is the biologically active isoform mediating the metabolic functions. However, several recent clinical studies have demonstrated that not only undercarboxylated but also total osteocalcin were associated with energy metabolism and atherosclerosis as well [
8,
9,
14]. Because lack of an automated assay to examine the uncarboxylated form, our study only measured serum total osteocalcin.
Acknowledgements
This work was funded by Project of National Natural Science Foundation of China (81170788), 973 Program of China (2012CB524906), National Key Technology R&D Program of China (2012BAI02B03), Grant from Shanghai Health and Family Planning Commission (2013ZYJB1001), and Key Project of Science and Technology of Shanghai (13XD1403000).
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
YB and WJ conceived and designed the experiments. YL, XM, YH, QX, YX performed the experiments. XP provided technical support. YL performed statistical analysis and wrote the paper. YL and XM contributed equally to this work and were the guarantors. All authors read and approved the final manuscript.