Importance of HIF-1 and HIF-2 for hypoxia-induced EPO expression in REPCs. (A) HIF-1α and HIF-2α proteins were knocked down by siRNA treatment, and the expression of the respective mRNAs was analyzed by real-time PCR and normalized to β-actin. Shown are the means ± SD; n = 6; *P < .05 with respect to the nontarget HIF-1α control; #P < .05 with respect to the nontarget HIF-2α control. (B) Effects of HIF-1α and HIF-2α siRNA treatment on the accumulation of HIF proteins under normoxic and hypoxic conditions. Shown are representative immunoblots for HIF-1α and HIF-2α. α-Tubulin served as the loading control. (C) Knockdown of HIF-1α and HIF-2α exerted different effects on the normoxic and hypoxia-induced EPO expression. Normoxic EPO expression was down-regulated by knockdown of HIF-1α and HIF-2α in a comparable manner. Hypoxia-induced EPO expression was more sensitive toward HIF-2α knockdown. EPO mRNA was quantitated by real-time PCR and normalized to β-actin. Shown are the means ± SD; n = 6, *P < .05; #P < .05; **P < .01.