Hypoxia-responsive miRNA-21-5p inhibits Runx2 suppression by targeting SMAD7 in MC3T3-E1 cells

Sustained hypoxia inhibits osteogenesis and osteoblast differentiation by downregulating the expression of runt-related transcription factor 2 (Runx2). MicroRNAs (miRNAs) have been shown to regulate osteogenesis and osteoblast differentiation. In the present study, we profiled miRNAs, with microRNA array and quantitative real-time polymerase chain reaction (RT-PCR) methods, in mouse osteoblast (MC3T3-E1) cells under hypoxia. Then, we investigated regulation by miRNA-21-5p on the expression of Runx2 and other osteoblast differentiation-associated markers via gain-of-function and loss-of-function strategies. We found that expression of miRNA-21-5p, miRNA-210-5p, and other eight miRNAs was upregulated significantly in hypoxia-treated MC3T3-E1 cells. miRNA-21-5p overexpression downregulated the expression of the mRNA and protein of suppressor of mothers against decapentaplegic (SMAD7) markedly, the 3 '-untranslated region (3 '-UTR) of which was highly homologous with the miRNA-21-5p sequence. miRNA-21-5p overexpression upregulated the protein expression of Runx2 in hypoxia-treated MC3T3-E1 cells, although mRNA expression of Runx2 and other osteoblast differentiation-associated molecules (eg, osteocalcin, procollagen type 1 amino-terminal propeptide, P1NP) were not regulated by it; such upregulation was SMAD7-dependent. In conclusion, hypoxia-responsive miRNA-21-5p promoted Runx2 expression (at least in part) by targeting the 3 '-UTR and downregulating SMAD7 expression. Our study suggests a protective role of miRNA-21-5p in promoting osteoblast differentiation under hypoxia.