In this study, we found that serum osteocalcin levels showed association with the risk of carotid atherosclerosis from the cross-sectional data in type 2 diabetes. Moreover, these associations are independent of lifestyle factors, duration of diabetes, family history of diabetes, and, remarkably, CRP, HbA1c, HOMA-IR and BMI.
Serum osteocalcin correlated negatively with TG and TC in our study, which is consistent with previous studies. It is well established that elevated serum TC was an independent risk factor for the development of CVD. Given that serum osteocalcin levels were significantly associated with CRP and TC levels, it is plausible to consider osteocalcin as a promising candidate for risk assessment and a potential intervention target for CVD. In a Japanese study, serum osteocalcin was negatively associated with intima-media thickness of common carotid artery in type 2 diabetic men . In another study, serum levels of osteocalcin were inversely associated with the metabolic syndrome and the severity of coronary artery disease in Chinese men . Moreover, an Australian study showed that serum osteocalcin levels predicted all-cause and CVD-related mortality in community-dwelling older men . Interestingly, in line with our hypothesis, we observed that serum osteocalcin levels is an independent risk factor for carotid atherosclerosis in patients with type 2 diabetes. Certainly, prospective studies with solid clinical end points are urgently needed to clarify whether low osteocalcin level plays a causal role in the development of atherosclerosis.
Endothelial inflammation plays a major role in the development of cardiovascular disease (CVD) . The inflammatory process is complex, with various cellular mediators known to contribute . We further tested the correlation between chronic low-grade inflammation and osteocalcin. In our study, serum osteocalcin correlated negatively with CRP. CRP is a pattern-recognition molecule of innate immunity as an acute-phase reactant and a hallmark of low-grade systemic inflammation [23, 24]. CRP is a well known serum marker of chronic low-grade inflammation and has been associated with diabetes, hypertension and CVDs. Hence, the results of current study might be partially mediated via the mechanism of chronic low-grade inflammation.
Moreover, there is evidence to show the influence of bone proteins on cardiovascular disease [25–33]. During atherogenesis, bone matrix proteins, including osteocalcin, may have a regulatory role in the atherosclerotic calcification process . Recent evidence suggests that osteoblast-like cells are present in the vasculature and capable of calcifying vascular cells. Furthermore, paracrine regulators of bone metabolism such as osteocalcin, matrix Gla protein (MGP), osteopontin, and bone morphogenetic protein are also present in atherosclerotic arteries. Thus, the vascular microenvironment possesses mechanisms similar to those in bone tissues to maintain mineral homeostasis. Both MGP and osteocalcin are known to be Gla-containing proteins. MGP is a secretory protein with widespread tissue expression, including in bone and vascular walls. MGP-knockout mice develop extensive calcification of arteries that rapidly becomes lethal, suggesting that MGP has an antimineralization role in the artery. In humans, osteocalcin is expressed parallel to MGP in both normal and atherosclerotic vessels and is also detected in human carotid arteries from endarterectomy samples. Thus, the two Gla proteins, osteocalcin and MGP, could play a pivotal role in not only bone mineralization but also vascular wall calcification . However, at present, little is known about whether serum osteocalcin secreted from osteoblasts in bone or osteoblast-like cells in vessels actually could modulate atherosclerosis. Thus, further studies are needed to clarify the pathophysiological processes underlying the relationship between serum osteocalcin level and atherosclerosis parameters.
The circulating measure of total osteocalcin includes both carboxylated and uncarboxylated forms . Animal and in vitro data implied that only the uncarboxylated form of osteocalcin functioned hormonally in the regulation of glucose homeostasis and energy metabolism. The latest researches based on population showed that elevated levels of both carboxylated and uncarboxylated forms of osteocalcin were associated with improved glucose tolerance; but the uncarboxylated form was related to insulin secretion, and the carboxylated form was associated with insulin resistance. Our study only measured total osteocalcin and did not have measurements of uncarboxylated osteocalcin; therefore, we could not further verify this hypothesis. It may be of interest to elucidate this potential mechanism in the context of the uncarboxylated and carboxylated forms and determine the direction of causality in a further longitudinal study.
Strengths of this study include the large size of the cohort, the use of early morning blood samples to minimise any effect of diurnal variation on TOC levels and the storage of aliquots at −80°C until assay. We excluded men who had conditions known to affect osteocalcin levels namely bone fracture, Paget’s disease and use of bisphosphonate or glucocorticoid therapy. However, As a cross-sectional study, there are several limitations. The mechanisms underlying these associations are still to be explored. The present findings are inherently limited in the ability to eliminate causal relationships between osteocalcin and atherosclerosis. Although most potential confounders were carefully controlled, since some of the study population had several risk factors including hypertension, and dyslipidemia, we could not eliminate the possible effect of underlying diseases and medications used for these diseases on the present findings. Further prospective population-based studies are needed to investigate the mechanisms in order to answer these questions.
In conclusion, our study indicated that serum osteocalcin levels were significantly associated with carotid atherosclerosis in patients with type 2 diabetes, even after adjustment for other potential confounders. This may reflect the role of osteocalcin as a circulating endocrine factor which regulates glucose metabolism and thereby cardiovascular risk in patients with type 2 diabetes. Prospective studies are needed to assess the time course and relevance of serum osteocalcin in the development of atherosclerosis in patients with type 2 diabetes.