TIEG1 is upregulated in Lrp5/6-mediated valve osteogenesis.

We have previously demonstrated that Lrp5/6/β-catenin plays an important role in valve calcification with a specific osteogenic phenotype defined by increased bone mineral content and overall valve thickening. Recent studies indicate that TIEG1 may be involved in mediating the Wnt signaling pathway in bone, which is known to play critical roles in osteoblast differentiation and bone mineralization. Therefore, we sought to test the role of TIEG1 in mediating Wnt signaling, in an established model of hypercholesterolemic valve disease. Our previous model treated null mice with cholesterol diets: Lrp5 -/- /ApoE -/- mice versus wild-type control (n = 180). Group I (n = 60) normal diet, Group II (n = 60) 0.25% chol diet (w/w), and Group III (n = 60) 0.25% (w/w) chol diet + atorv was tested for gene expression for TIEG1, Lrp6, and Runx2. Real-time polymerase chain reaction confirmed that there is upregulation of the gene expression for TIEG1 and Runx2 in the hypercholesterolemic double knockout and single knockout valves as compared with controls with a mild increase in Lrp6. To confirm the mechanism, coexpression of β-catenin, TIEG1, and LEF1 in valve cells in vitro, led to the coactivation of the TOPFLASH reporter, which was further confirmed by the observation that TIEG1 and β-catenin colocalize with one another in the nucleus of valvular interstitial cells (VICs) following stimulation with transforming growth factor-β treatment, an established activator of TIEG1. Taken together, these data implicate an important role for TIEG1 in mediating valve osteogenesis.