Enhanced Mineralization Potential of Vascular Cells from SM22α-Rankl ^tg Mice

Vascular calcification, prevalent in diabetes and chronic kidney disease, contributes to morbidity and mortality. To investigate the effect of receptor activator of NF-kB ligand (RANKL) on vascular calcification in vivo, transgenic mice, where RANKL expression was targeted to vascular smooth muscle cells using the SM22α promoter (SM22α-Rankl ^tg ), were created. Sixteen-month-old male SM22α-Rankl ^tg mice had higher body weight and higher serum calcium levels but lower lumbar bone mineral density (BMD) compared with age- and gender-matched wild-type (WT) littermates. BMD of long bones, body fat (percent of weight) of the leg, and serum levels of phosphate and RANKL were not significantly different. No significant differences in these parameters were observed in female mice. Histological analysis did not reveal calcium deposits in the aortic roots of SM22α-Rankl ^tg mice. To analyze the osteoblastic differentiation and mineralization potentials of vascular cells, aortic smooth muscle cells (SMCs) were isolated and cultured. Results showed that SM22α-Rankl ^tg SMCs had higher baseline alkaline phosphatase (ALP) activity but not baseline matrix calcification. When induced by the PKA agonist forskolin, ALP activity was greater in SM22α-Rankl ^tg than in WT SMCs. Real-time RT-qPCR revealed higher baseline expression of ALP and ankylosis genes but lower osteoprotegerin gene in SM22α-Rankl ^tg SMCs. Matrix mineralization induced by inorganic phosphate or forskolin was greater in SM22α-Rankl ^tg than in WT SMCs. Treatment of these cells with the ALP inhibitor levamisole abolished forskolin-induced matrix mineralization but not inorganic phosphate-induced matrix mineralization. These findings suggest that RANKL overexpression in the vasculature may promote mineralization potential.