Expression of NPP1 is regulated during atheromatous plaque calcification

Mutations of the ENPP1 gene encoding ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) are associated with medial calcification in infancy. While the inhibitory role of matrix proteins such as osteopontin (OPN) with respect to atherosclerotic plaque calcification has been established, the role of NPP1 in plaque calcification is not known. We assessed the degree of plaque calcification (computed tomography), NPP1 and OPN localization (immunohistochemistry) and expression (RT-PCR) in a cohort of 45 patients undergoing carotid endatherectomy for significant stenosis of the internal carotid artery and in normal arteries (N= 50). We correlated NPP1 and OPN expression levels to the degree of plaque calcification, to pro-atherogenic factors and statin therapy. NPP1 was demonstrated in the base and in the shoulder of atherosclerotic plaques. Compared to normal arteries and non-calcified plaques, in calcified plaques NPP1 mRNA was decreased (P < 0.0001). OPN mRNA levels were up-regulated in carotid atheroma. NPP1 and OPN expression levels positively correlated with the degree of plaque calcification (R= 0.54, P= 0.00019 and R= 0.46, P= 0.017, respectively) and with risk factors of atherosclerosis. Expression of the calcification inhibitor NPP1 is down-regulated in calcified atherosclerotic plaques. Our correlation data point to a counter-active mechanism, which in the end turns out to be insufficient to prevent further progression of calcification.