Hydrolysis of Extracellular ATP by Vascular Smooth Muscle Cells Transdifferentiated into Chondrocytes Generates P(i) but Not PP(i)

(1) Background: Tissue non-specific alkaline phosphatase (TNAP) is suspected to induce atherosclerosis plaque calcification. TNAP, during physiological mineralization, hydrolyzes the mineralization inhibitor inorganic pyrophosphate (PP(i)). Since atherosclerosis plaques are characterized by the presence of necrotic cells that probably release supraphysiological concentrations of ATP, we explored whether this extracellular adenosine triphosphate (ATP) is hydrolyzed into the mineralization inhibitor PP(i) or the mineralization stimulator inorganic phosphate (P(i)), and whether TNAP is involved. (2) Methods: Murine aortic smooth muscle cell line (MOVAS cells) were transdifferentiated into chondrocyte-like cells in calcifying medium, containing ascorbic acid and β-glycerophosphate. ATP hydrolysis rates were determined in extracellular medium extracted from MOVAS cultures during their transdifferentiation, using (31)P-NMR and IR spectroscopy. (3) Results: ATP and PP(i) hydrolysis by MOVAS cells increased during transdifferentiation. ATP hydrolysis was sequential, yielding adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine without any detectable PP(i). The addition of levamisole partially inhibited ATP hydrolysis, indicating that TNAP and other types of ectonucleoside triphoshatediphosphohydrolases contributed to ATP hydrolysis. (4) Conclusions: Our findings suggest that high ATP levels released by cells in proximity to vascular smooth muscle cells (VSMCs) in atherosclerosis plaques generate P(i) and not PP(i), which may exacerbate plaque calcification.