We have observed that GADD34 expression counterbalances PKR activation by promoting eIF2a dephosphorylation, however it has little impact on reversing the global translation inhibition initiated by poly I:C. We next monitored the production of specific proteins and cytokines in WT and GADD34 DC/DC MEFs (Fig. 4) . . PKR is required for ATF4 and GADD34 expression in response to poly I:C. A) WT and PKR 2/2 MEFs were stimulated for 8 h with poly I:C (pI:C), or the UPR-inducing drug, thapsigargin (th) for 6 h. ATF4 protein expression was detected by immunoblot on nuclear extracts. Nuclear HDAC1 immunoblot is shown for equal loading control. * indicates unspecific band. B) GADD34 mRNA levels were quantified by qPCR after 6 h of poly I:C (pI:C) treatment in WT and PKR 2/2 MEFs. For the same cell extracts, immunoblots of GADD34 (middle panel), PKR and P-eIF2a (right panel) were performed. C) The same analysis was performed in eIF2a A/A and control eIF2a S/S MEFs. Treatment with thapsigargin (th), for 6 h was used as control to induce GADD34 and P-eIF2a. eIF2a and b-actin immunoblots are shown for equal loading control. Quantitative PCR data are the mean 6 standard deviation of 3 independent experiments. doi:10.1371/journal.ppat.1002708.g002