Elastin degradation accelerates phosphate-induced mineralization of vascular smooth muscle cells

Medial layer vascular calcification is common in patients with end-stage kidney disease. Inorganic phosphate has been shown to accelerate the transformation of vascular smooth muscle cells (VSMCs) into osteoblast-like cells, which is thought to be a major process of medial layer calcification. Although elastin degradation is associated with medial layer calcification, the linkage between elastin degradation and the transformation of VSMCs remains to be clarified. We investigated the involvement of elastin degradation in the transformation of VSMCs. Rat VSMCs were isolated and cultured with a normal- (NP, 1.0 mM) or high- (HP, 2.5 mM) phosphate medium. An elastin-derived peptide, alpha-elastin (500 microg/ml), was also added to the normal- (NP + E) or high- (HP + E) phosphate medium. After a culture period of 2 weeks, von Kossa staining revealed mineralization in the HP group, which was accelerated by alpha-elastin, whereas alpha-elastin did not affect the mineralization at a normal phosphate concentration. The gene expression of osteoblastic differentiation factors, i.e., Runx2 or osteocalcin (OC), in VSMCs was significantly increased in the HP (Runx2 P < 0.05, OC P < 0.05) and HP + E (OC P < 0.05) groups compared with the NP and NP + E groups. Both gene and protein expressions of tissue-nonspecific alkaline phosphatase (TNAP) were significantly increased in the HP group compared with the NP and NP + E groups (P < 0.01, respectively). This increment was augmented in the HP + E group (P < 0.01). These results suggest that elastin degradation would accelerate or stabilize the process of VSMC transformation, which is induced by high phosphate through the upregulation of TNAP.