The results of the study demonstrate that vascular calcification is closely associated with serum osteoprotegerin levels in patients with diabetes, but is not linked to other bone-related humoral factors including osteocalcin, FGF23, and 25-hydroxyvitamin D3. Osteoprotegerin may directly affect osteoblastic changes of vascular smooth muscle cells and is not mediated through other factors or ions[12, 13]. This differs from the effect of FGF23 on vascular calcification, since FGF23 regulates phosphate metabolism in kidney and promotes vascular calcification in association with phosphate. In this study, we examined diabetic patients with mild or moderate renal impairment, including subjects with diabetic nephropathy of stages 1–4, since patients with advanced nephropathy may have vascular calcification without any interaction with cytokines.
FMD is a good indicator for vascular endothelial function and was found to be unrelated to serum osteoprotegerin or vascular calcification. These findings indicate that elevated serum osteoprotegerin may be involved in vascular calcification in patients with diabetes, independently of progression of diabetic nephropathy. However, because the study was performed as a cross-sectional observation, the finding of an association of serum osteoprotegerin and vascular calcification is limited and cannot suggest causality. However, osteoprotegerin may be clinically useful as a biochemical marker of vascular damage and overall burden of atherosclerotic disorders. In fact, serum osteoprotegerin is known to be associated with carotid and peripheral arterial disease in patients with type 2 diabetes and is inversely associated with aortic distensibility. Osteoprotegerin is also an independent predictor of coronary artery disease in asymptomatic type 2 diabetic patients with microalbuminuria and is also predictive of the long-term outcome in patients with ST-elevation myocardial infarction treated with percutaneous coronary intervention.
Osteoprotegerin is a member of the TNF-related family and exerts its major biological action through binding to RANKL as a soluble decoy receptor, with resulting inhibition of RANK stimulation of osteoclast differentiation and bone resorption[13, 25]. RANKL and osteoprotegerin are expressed in osteoblasts, and the receptor RANK is expressed in osteoclasts cells. In vascular beds, endothelial cells and vascular smooth muscle cells produce osteoprotegerin, but do not produce RANK and RANKL[11, 13, 26, 27]. However, both RANKL and RANK expression have been detected in atherosclerotic lesions[28–30]. This may indicate that the RANK-RANKL interaction induces osteoclast formation and that osteoprotegerin blocks this interaction to reduce arterial calcification[11, 31, 32]. Therefore, vascular calcification linked to RANK-RANKL is independent of advanced vascular damage related to a long duration of diabetes, progression of diabetic microvascular complications, and abnormal metabolism of calcium or phosphate associated with FGF23, 25-hydroxyvitamin D3 and other factors. In the present study, we could not determine how the RANK/RANKL system participates in development of osteoclast differentiation in the vasculature in the non-advanced stage of diabetes mellitus. However, our findings suggest that vascular calcification may develop in patients with diabetes with high serum osteoprotegerin levels, even if microvascular and macrovascular disorders do not become manifest. The elevation of serum osteoprotegerin might be associated with underlying alterations in RANK/RANKL interactions in the vascular wall.
Vascular calcification is common in patients with diabetes, and especially in those with diabetic nephropathy[8–10]. We have shown that coronary artery calcification is significantly increased in advanced diabetic nephropathy based on histology of intravascular ultrasound. In such cases, abnormal calcium or phosphorus metabolism may also be involved in vascular calcification, in association with FGF23 or 25-hydroxyvitamin D3[20, 33]. As shown in Table3, we found no relationship of vascular calcification or serum osteoprotegerin levels with diabetic nephropathy, and only 6 of the 124 subjects in the study had stage 4 diabetic nephropathy. Thus, most had no or mild nephropathy, making it evident that progression of vascular calcification is not simply associated with diabetic nephropathy.
BMI, systolic blood pressure, adiponectin and 25-hydroxyvitamin D3 were among the factors correlated with serum osteoprotegerin in simple linear regression analysis. There was a highly significant positive correlation between serum adiponectin and serum osteoprotegerin. In bone tissue, adiponectin stimulates RANKL in osteoclasts, thus inducing osteoclastogenesis. In contrast, adiponectin inhibits osteoprotegerin in osteoblasts. The positive correlation between serum adiponectin and osteoprotegerin indicates that serum adiponectin may interact with osteoprotegerin for modulating osteoclast formation. Serum osteoprotegerin had a negative correlation with serum 25-hydroxyvitamin D3. Several studies have examined the effect of vitamin D3 on osteoprotegerin synthesis and it has been shown that active 1α, 25-hydroxyvitamin D3 upregulates the level of RANKL, but downregulates osteoprotegerin expression in human periodontal ligament cells by accelerating degradation of osteoprotegerin mRNA and transrepressing the osteoprotegerin gene[37, 38]. This indicates that active vitamin D3 promotes osteoclastogenesis. Similar interactions of osteoprotegerin with adiponectin and 25-hydroxyvitamin D3 may occur in vascular tissues, particularly with regard to the vascular calcification evaluated in this study.