The present study is the first to show that in patients with type 2 diabetes and normal or slightly altered kidney function, dp-ucMGP levels (a marker for vitamin K status) are independently associated with age, eGFR and VKA treatment. More importantly, peripheral VC is associated with dp-ucMGP (independently of age, gender, previous CVD and t-ucMGP levels).
Matrix Gla protein is a strong inhibitor of VC, as revealed by the development of massive VC in MGP knockout mice . Indeed, MGP is a vitamin K-dependent inhibitor of calcium phosphate precipitation and crystal formation in the vessel wall . Furthermore, it suppresses the activity of bone morphogenetic proteins 2 and 4 [22, 23]. The value of MGP as a calcification biomarker has been evaluated in various cohorts, and a number of assays have been developed to measure its various conformations. In the present study, we evaluated circulating t-ucMGP and dp-ucMGP concentrations. We focused on dp-ucMGP because this form has low affinity for vascular calcium deposits and is secreted into the bloodstream by vascular smooth muscle cells. Moreover, dp-ucMGP is a well-known marker for vascular vitamin K status. High levels of dp-ucMGP have been associated with aortic calcification (independently of classical risk factors) in patients at different stages of CKD [10, 24]. Similarly, the present study is the first to demonstrate that in patients with type 2 diabetes and normal or slightly altered kidney function, dp-ucMGP levels were positively associated with peripheral artery calcification (independently of age, gender, previous CVD and t-ucMGP). It is important to study VC in diabetic patients, since as calcification process could present specific features. Indeed Flammer et al. recently reported that patients with elevated HbA1c levels had a significantly higher percentage of circulating blood mononuclear cells expressing the osteoblastic marker osteocalcin. However, further research is required to establish whether these cells increase VC . On the same lines, a distinct subpopulation of circulating cells expressing osteocalcin and bone alkaline phosphatase had procalcific activity in type 2 diabetic patients .
Indeed, the present study is the first to have evaluated the potential role of this important calcification inhibitor and its relationship with peripheral artery calcification (as evaluated with a CT-based methodology). This result is in agreement with recently published data from a prospective study of 518 patients with type 2 diabetes (mean follow-up period: 11.2 years); Dalmeijer et al. demonstrated a strong, independent relationship between high dp-ucMGP and cardiovascular risk (and PAD in particular), whereas t-ucMGP levels were not associated with CVD . Indeed, the presence of medial calcification (which is particularly present in peripheral arteries ) could explain the positive correlation between dp-ucMGP levels and PAD.
Only one study (in outpatients with stable CVD) reported that higher t-ucMGP levels are associated with lower mitral annular calcification (MAC) in persons without diabetes and higher MAC in persons with diabetes . In the present study, a univariate analysis revealed an inverse relationship between t-ucMGP and peripheral artery calcification; however, this link was no longer significant after adjustment for dp-ucMGP. The conflicting results in these two studies might be due to differences in the calcification site evaluated (MAC versus peripheral calcification) and patient recruitment (outpatients with stable CVD versus patients in a diabetes ward). However, it appears to be important to assay for dp-ucMGP when focusing on peripheral VC, since this factor is still associated with peripheral artery calcification after multiple adjustments. Furthermore, Dalmeijer et al. reported that dp-ucMGP levels (but not t-ucMGP levels) were associated with PAD. These findings suggest that VC sites differ in terms of their specific features and biomarkers. This is of particular interest in the pathophysiology of VC in patients with diabetes, who are particularly prone to this condition. Indeed, peripheral artery calcification is very frequent in patients with diabetes and can lead to amputation.
Furthermore, we identified eGFR, age and current VKA use as independent factors for dp-ucMGP levels. It is already know that in patients with CKD, levels of inactive forms of MGP increase progressively . Even when patients with normal or slightly altered kidney function are selected, eGFR is still a strong predictor of MGP levels. It is noteworthy that the link between dp-ucMGP levels and calcification appeared to be independent of kidney function in the present study.
The use of VKAs appears to be an important predictor of dp-ucMGP; the six VKA-treated patients had dp-ucMGP levels that were around 4 times greater than in the other patients. Indeed, VKA treatment indirectly inhibits the carboxylation of vitamin K-dependent proteins via interactions with vitamin K epoxide reductase. Hence, VKA treatment inhibits the gamma-carboxylation of MGP and leads to an increase in inactive forms of MGP and thus prompts VC [28, 29]. Indeed, epidemiological studies performed in the last decade have revealed VC in warfarin-treated patients .
Until recently, VKAs were the only drugs for long-term treatment of thromboembolic disorders. However, novel new oral anticoagulant agents (NOACs) have now emerged (e.g. factor Xa inhibitors such as rivaroxaban, apixaban and factor IIa inhibitors such as dabigatran). Unlike VKAs, the NOACs do not interfere with vitamin K-dependent proteins and may thus be safer with regard to VC. This advantage is particular important in diabetic populations, who are particular prone to the development of VC. However, this hypothesis needs to be confirmed in prospective trials and balanced against the efficacy and safety of NOACs.
Given that VC in general and peripheral calcification in particular are major problems in patients with diabetes, modulation of vitamin K status might be an interesting therapeutic option. Indeed, vitamin K is gaining increasing attention in terms of its therapeutic potential in VC [30, 31]. In a recent pilot study, vitamin K supplementation in dialyzed patients was tested as a means of improving vitamin K status. Indeed, short-term supplementation with menaquinone-7 (vitamin K2) was found to reduce dp-ucMGP levels in haemodialysis patients . It remains to be seen whether vitamin K supplementation could have an impact on VC in patients with diabetes (through evaluation in large clinical trials).
The limitation of the present study included the small sample size, the lack of evaluation of vitamin K intake and serum vitamin K levels, and the evaluation of marker levels at a single time point. The study would have been strengthened by the presence of a control group of participants with vascular calcification data, so that they could have been compared with diabetic patients. In contrast, one of the study’s main strengths relates to the fact that this was the first study to concomitant evaluate dp-ucMGP/t-ucMGP and peripheral artery calcification in patients with type 2 diabetes.