Expression of HIF-1α and miR-21 in periodontal ligament cells under hypoxia. (A) Expression of HIF-1α mRNA was rapidly and significantly increased under hypoxia at 6 h compared with the normoxia group, then the expression of HIF-1α mRNA slowly decreased. (B) miR-21 was significantly increased in periodontal ligament cells when exposed to hypoxia and remained significantly higher under hypoxia from 6 to 48 h compared with the normoxia group. (C and D) HIF-1α protein exhibited significantly higher expression at 6 and 12 h, and also decreased slowly (*P<0.05, **P<0.01 vs. the 48 h group). HIF-1α, hypoxia-inducible factor-1α; miR, microRNA.