Inhibition of PKCδ blocks ATRA-induced PKR and Ser51-phosphorylation of eIF2α. (a) Inhibition of PKCδ blocked ATRA-induced phosphorylation (Thr446)/activation of PKR. NB4 cells were pretreated with PKCδ inhibitor rottlerin (4 μM) for 4 h before adding ATRA (1 μM). (b) Rottlerin (4 M) inhibits ATRA-induced expression of PKCδ in NB4 cells detected by Western blot analysis. (c) Knockdown of PKCδ by a specific siRNA results in reduction of phosphorylation (Thr446) of PKR in NB4 cells. Bar graph represents inhibition of relative expression of PKCδ (PKCδ/β-actin) and p-PKR (p-PKR/β-actin) based on densitometric analysis of Western blots.