Chondrocyte rather than osteoblast conversion of vascular cells underlies medial calcification in uremic rats.

OBJECTIVE To investigate cell biological changes in calcified aortas of rats that experienced chronic renal failure. METHODS AND RESULTS Vascular smooth muscle cells have the potential to transdifferentiate to either chondrocytes or osteoblasts, depending on the molecular pathways that are stimulated. Uremia-related medial calcification was induced by feeding rats an adenine low-protein diet for 4 weeks. Aortic calcification was evaluated biochemically and histochemically and with in vivo micro-computed tomographic scanning. Immunohistochemistry and RT-PCR were applied to analyze the time-dependent aortic expression of molecules involved in the segregation between the chondrocyte versus osteoblast differentiation pathway. After 4 weeks, 85% of the uremic rats had developed distinct aortic medial calcification, which increased to severely calcified lesions during further follow-up. The calcification process was accompanied by a significant time-dependent increase in the expression of the chondrocyte-specific markers sex determining region Y-box 9 (sox9), collagen II, and aggrecan and a nonsignificant trend toward enhanced core binding factor alpha 1 (cbfa1), and collagen I. The expression of the osteoblast marker osterix and both lipoprotein receptor-related protein 6 and beta-catenin, molecules of the wingless-type MMTV integration site family member (Wnt)/beta-catenin pathway induced during osteoblast differentiation, was suppressed. CONCLUSIONS In the aorta of uremic rats, medial smooth muscle cells acquire a chondrocyte rather than osteoblast phenotype during the calcification process.