Complementary Roles of GADD34- and CReP-Containing Eukaryotic Initiation Factor 2α Phosphatases during the Unfolded Protein Response

Abstract

cPhosphorylation of eukaryotic initiation factor 2␣ (eIF2␣) controls transcriptome-wide changes in mRNA translation in stressed cells. While phosphorylated eIF2␣ (P-eIF2␣) attenuates global protein synthesis, mRNAs encoding stress proteins are more efficiently translated. Two eIF2␣ phosphatases, containing GADD34 and CReP, catalyze P-eIF2␣ dephosphorylation. The current view of GADD34, whose transcription is stress induced, is that it functions in a feedback loop to resolve cell stress. In contrast, CReP, which is constitutively expressed, controls basal P-eIF2␣ levels in unstressed cells. Our studies show that GADD34 drives substantial changes in mRNA translation in unstressed cells, particularly targeting the secretome. Following activation of the unfolded protein response (UPR), rapid translation of GADD34 mRNA occurs and GADD34 is essential for UPR progression. In the absence of GADD34, eIF2␣ phosphorylation is persistently enhanced and the UPR translational program is significantly attenuated. This "stalled" UPR is relieved by the subsequent activation of compensatory mechanisms that include AKT-mediated suppression of PKR-like kinase (PERK) and increased expression of CReP mRNA, partially restoring protein synthesis. Our studies highlight the coordinate regulation of UPR by the GADD34-and CReP-containing eIF2␣ phosphatases to control cell viability.

Developmentally regulated PERK activity renders dendritic cells insensitive to subtilase cytotoxin-induced integrated stress response

“…These results confirm that GADD34 is produced and active in steady-state DC, suggesting the existence of a basal IRF3-dependent Ppp1r15a mRNA transcription in DCs. Interestingly these results also imply that GADD34 mRNA translation occurs independently of any acute stress in DCs, despite its upstream uORF-dependent translational regulation, as phenomenon recently observed in MEFs (Reid et al, 2016).…”

PPP1R15A-mediated dephosphorylation of eIF2α is unaffected by Sephin1 or Guanabenz

“…Previous studies have emphasized the dominance of translational recovery in the physiological action of PPP1R15A, such that Ppp1r15a KO attenuates both the burden of protein misfolding (Marciniak et al, 2004) and the response to it (Reid et al, 2016). In keeping with these ideas and with the findings of Das and colleagues (Das et al, 2015), Sephin1 attenuated the activity of both UPR pathways in cells exposed to tunicamycin; an inhibitor of N-linked glycosylation, that promotes misfolding of newly-synthesized proteins (Figure 9A).…”

“…Moreover, when challenged with tunicamycin, which causes unfolded protein stress in the endoplasmic reticulum by inhibiting N-linked glycosylation, homozygous Ppp1r15a tm1Dron mice and cultured cells derived from them are relatively resistant to the toxin’s lethal effects (Marciniak et al, 2004; Reid et al, 2016). This feature is plausibly attributed to sustained activity of the ISR in the Ppp1r15a mutant mice, which favours proteostasis by limiting the production of unfolded proteins under stress conditions (Boyce et al, 2005; Han et al, 2013).…”

Functions and therapeutic potential of protein phosphatase 1: Insights from mouse genetics

“…The expression of GADD34 is mainly induced in stress conditions, while CReP is constitutively expressed and its levels are unchanged by stress [119][120][121]. Stress signaling in eukaryotic cells depends critically on the phosphorylation of eIF2α at serine 52, which occurs for example in response to the accumulation of misfolded proteins in the endoplasmic reticulum (also known as the unfolded protein response UPR), amino acid deprivation and viral infections [120,121]. Upon stress, eIF2α gets hyperphosphorylated at serine 52, attenuates global protein synthesis and induces the expression of GADD34 (but not CReP) at the transcript and protein levels.…”