Hydroxyapatite deposition on the medial layer of the aortic walls is the hallmark of vascular calcification and the most common complication in aging individuals and in patients with diabetes and those undergoing hemodialysis. Extracellular pyrophosphate is a potent physicochemical inhibitor of hydroxyapatite crystal formation. This study analyzed changes in extracellular pyrophosphate metabolism during the calcification process.
Pi-induced calcification of ex vivo-cultured aortic rings resulted in calcium accumulation after 7 days. This accumulation was enhanced when aortic walls were devitalized. BMP2 expression was associated with calcium accumulation in cultured aortic rings, as well as in cultured VSMCs and in calcitriol-induced calcification in rats. Hydroxyapatite dose-dependently induced BMP2 overexpression in VSMCs. Moreover, TNAP mRNA levels and activity were found to be downregulated in early phases and upregulated in later phases of calcification in all three models studied. eNPP1 increased from early to later phases of calcification, whereas eNTPD1 was downregulated during later phases. Synthesis of pyrophosphate in VSMCs increased significantly over time, in all three models studied. Because the rate of pyrophosphate hydrolysis was 10 times slower than the rate of pyrophosphate synthesis, pyrophosphate synthesis is determined mainly by the ratio of eNPP1 to eNTPD1 activity. Hydroxyapatite also induces increments both in TNAP and eNPP1/eNTPD1 ratio in VSMCs.
These findings indicate that pyrophosphate synthesis increases in VSMCs during early calcification. Moreover, BMP2 may be a good marker of calcification, especially during early phases. By contrast, TNAP expression is not a good marker of calcification due to its regulation by various stimuli.