Synergistic Inhibition of Endochondral Bone Formation by Silencing Hif1α and Runx2 in Trauma-induced Heterotopic Ossification

Angiogenesis and osteogenesis are tightly coupled during bone development. We studied the effect of inhibition of Hif1 alpha and Runt-related protein 2 (Runx2) on the formation of heterotopic ossification (HO). We constructed lentivirus vectors expressing Hif1 alpha small interfering RNA (siRNA) and Runx2 siRNA. The inhibition of Hif1 alpha function impaired osteoblast proliferation while osteoblasts differentiated normally. Osteoblasts lacking Runx2 proliferated normally while the differentiation was impaired. The osteoblast differentiation was significantly inhibited by co-Runx2 and Hif1 alpha siRNA treatment. The formation of HO by inhibiting Runx2 and Hif1 alpha in an animal model induced by Achilles tenotomy was investigated. The results showed that lacking of Runx2 and Hif1 alpha could inhibit HO formation. Inhibition of Hif1 alpha prevented HO formation only at the initial step and inhibition of Runx2 worked both at the initial step and after chondrogenesis. Angiogenesis and the expressions of osteogenic genes were downregulated in the Hif1 alpha siRNA group. We found synergistic inhibition of endochondral bone formation by silencing Hif1 alpha and Runx2. Our study provided new insight into the roles of Hif1 alpha and Runx2 during the processes of endochondral bone formation, and had important implications for the new therapeutic methods to inhibit HO or to enhance bone formation.