The double‐strand RNA‐dependent protein kinase PKR plays a significant role in a sustained ER stress‐induced apoptosis

Abstract

Sustained ER stress leads to apoptosis. However, the exact mechanism still remains to be elucidated. Here, we demonstrate that the double strand RNA-dependent protein kinase (PKR) is involved in the ER stress-mediated signaling pathway. ER stress rapidly activated PKR, inducing the phosphorylation of eIF2a, followed by the activation of the ATF4/CHOP pathway. ER-stress-mediated eIF2a/ATF4/CHOP signaling and associated cell death was markedly reduced by PKR knockdown. We also found that PKR activation was mediated by PACT, the expression of which was elevated by ER-stress. These results indicate that the ER-stress-mediated eIF2a/ATF4/CHOP/cell death pathway is, to some degree, dependent on PACT-mediated PKR activation apart from the PERK pathway.

PKR , a p53 target gene, plays a crucial role in the tumor-suppressor function of p53

“…The phosphorylation of eIF2␣ at Ser-51 leads to a significant reduction in protein synthesis, concomitant with induced expression of the basic leucine zipper (bZIP) regulator, activating transcription factor 4 (ATF4), and its target gene CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), resulting in caspase activation and cell apoptosis (27). In accordance with our recent report (28), enhancements of ATF4/CHOP, followed by caspase-mediated cleavage of poly(ADP-ribose) polymerase (PARP) (29) were detected readily in p53 ϩ/ϩ PKR ϩ/ϩ cells but were barely detectable in the p53 ϩ/ϩ PKR KD (sh-PKR) cells under conditions of DNA damage (Fig. 4G).…”

Roles of the translationally controlled tumour protein (TCTP) and the double-stranded RNA-dependent protein kinase, PKR, in cellular stress responses

“…In order to gain further insight into the role of PKR in regulating TCTP levels under proapoptotic conditions, we studied the regulation of TCTP levels in PKR-knockout cells under various stress conditions and observed PKR-dependent downregulation of the protein under Ca þ þ -stress conditions. An involvement of PKR in cellular reactions to Ca þ þ stress was suspected earlier (Brostrom and Brostrom, 1998), and it was recently demonstrated that ER stresses activate PKR (Lee et al, 2007). .…”

“…The translation of TCTP mRNA is specifically inhibited by PKR (Bommer et al, 2002); this may provide an explanation for the PKR dependence of the regulation of TCTP in cells in response to thapsigargin and A23187 (Figure 1). However, the relative lack of effect of sodium arsenite and tunicamycin (Figures 1 and 2), which also promote eIF2a phosphorylation (Lu et al, 2001;Lee et al, 2007), suggests that phosphorylation of eIF2a, although necessary, is not sufficient for TCTP downregulation.…”

“…A major role for PKR has recently emerged in the control of apoptosis, following activation of the tumour-suppressor p53 pathway (Yoon et al, 2009). Inhibition of protein synthesis through eIF2a phosphorylation plays a considerable part in PKR activity (Scheuner et al, 2006;Lee et al, 2007), but other pathways are most likely also involved (Garcia et al, 2006).…”

“…The discovery of the PKR-like ER kinase (PERK) (Harding et al, 1999) has cast doubts on the involvement of PKR in the inhibition of protein synthesis caused by depletion of ER calcium stores. However, it has recently been established that PKR is activated in addition to PERK upon induction of ER stress by both thapsigargin and tunicamycin (Onuki et al, 2004;Lee et al, 2007;Singh et al, 2009).…”