1,25-Dihydroxyvitamin D3-induced aortic calcifications in experimental uremia: up-regulation of osteoblast markers, calcium-transporting proteins and osterix

Background and objective: Whether treatment with vitamin D receptor activators contributes to cardiovascular disease in patients with chronic kidney disease is a matter of debate. We studied mechanisms involved in vitamin D-related vascular calcifications in vivo and in vitro. Methods: Aortic calcifications were induced in subtotally nephrectomized (SNX) rats by treatment with a high dose (0.25 [mu]g/kg per day) of 1,25-dihydroxyvitamin D3 (calcitriol) given for 6 weeks. Likewise, primary rat vascular smooth muscle cells (VSMCs) were incubated with calcitriol at concentrations ranging from 10-11 to 10-7 mol/l. Immunohistochemistry revealed that the aortic expression of osteopontin, osteocalcin and bone sialoprotein was significantly increased in calcitriol-treated SNX rats compared to untreated SNX controls. In addition, aortic expression of the transient receptor potential vanilloid calcium channel 6 (TRPV6) and calbindin D9k was significantly up-regulated by treatment with calcitriol. Furthermore, calcitriol significantly increased expression of the osteogenic transcription factor osterix. In-vitro studies showed similar results, confirming that these effects could be attributed to treatment with calcitriol. Conclusions: High-dose calcitriol treatment induces an osteoblastic phenotype in VSMC both in SNX rats and in vitro, associated with up-regulation of proteins regulating mineralization and calcium transport, and of the osteogenic transcription factor osterix.