The role of calciprotein particles
Prevention of calcification is also a rather complex process involving Fetuin-A, a liver-derived blood protein that acts as a potent inhibitor of ectopic mineralization. Fetuin-A stabilizes supersaturated mineral solutions by forming soluble colloidal nanospheres termed calciprotein particles (CPP), in analogy to lipoprotein particles (8). They were first observed in haemodialysis patients a decade ago in a study which revealed that free Fetuin-A circulates free and does not precipitate, while under extra-osseous calcification stress Fetuin-A forms particles to inhibit mineral precipitation (9). These particles occur substantially where bone resorption releases high levels of calcium and phosphate, or when mineral homeostasis is severely disturbed, as in dialysis patients (9). Calciprotein particles in the interstitial spaces are cleared by monocytes, macrophages or other endocytosing cells. However, it has been hypothesized that overly abundant particles may overcome the clearing capacity and result in apoptosis of macrophages, endothelial cells and smooth muscle cells, and deposition of calcified apoptotic fragments (10).
Pasch et al. recently developed an assay for measuring extra-osseous calcification propensity by detecting spontaneous transition of spherical colloidal primary CPP to secondary elongate crystalline ones in the presence of artificially elevated calcium and phosphate concentrations (11). The test was named T50 in reference to the half-time needed for the maturation from primary to secondary CPP. The assay was validated in vivo using the sera from heavily calcified Fetuin-A–deficient, and age-matched noncalcified heterozygous, and wild-type mice, as well as on blood samples from haemodialysis patients and healthy volunteers. It showed excellent ability to discriminate the calcification-prone from the noncalcification-prone individuals (11). In transplant patients, reduced serum T50 was associated with increased risk of all-cause mortality, cardiovascular mortality, and graft failure (12). Furthermore, serum magnesium, albumin and parathyroid hormone levels were positively associated, and phosphate, haemoglobin, and the use of calcineurin inhibitors or vitamin K antagonists were inversely associated with serum T50 in transplanted patients (12). In a randomized, double-blind, placebo-controlled EVOLVE clinical trial, which recruited nearly four thousand haemodialysis patients with moderate to severe secondary hyperparathyroidism, lower T50 was associated with higher probability of cardiovascular events and mortality (13, 14).
The role of magnesium
In the recent years, several studies have demonstrated association between low magnesium levels and increased risk of vascular calcification, leading to the hypothesis that magnesium might counteract the development of calcifications. To test this assumption, a group of authors treated vascular smooth muscle cells with high phosphate or secondary CPP and supplemented with magnesium. The experiment confirmed that secondary CPP mediate phosphate-induced vascular calcifications, but also revealed that magnesium delays CPP maturation and therefore prevents calcification in a dose dependent manner (15). These in vitro results only confirmed previous observation that increasing dialysate magnesium increases T50 and hence, decreases calcification propensity in haemodialysis patients (16).
Calciphylaxis and novel treatments
Calciphylaxis is another important issue related to vascular calcification. It is a rare and serious condition caused by calcific occlusion of micro vessels in the subcutaneous adipose tissue and dermis that results in ischemic skin lesions (17). It is associated with severe pain sensation and poor survival. Even though multiple risk factors have been identified, the pathogenesis of calciphylaxis remains elusive. Multiple factors seem to be involved in this process, including calcification promoters and inhibitors (e.g., carboxylated matrix Gla protein, Fetuin-A, pyrophosphate and phytate) (17).