Examination of a second node of translational control in the unfolded protein response

Abstract

SummaryThe unfolded protein response (UPR) is a largely cytoprotective signaling cascade that acts to re-establish homeostasis of the endoplasmic reticulum (ER) under conditions of stress by inducing an early and transient block in general protein synthesis and by increasing the folding and degradative capacity of the cell through an extensive transcriptional program. It is well established that the mechanism for the early translational attenuation during ER stress occurs through phosphorylation of eukaryotic initiation factor 2 a (eIF2a) by activated PERK. Our data demonstrate that when eIF2a is dephosphorylated translation is not fully restored to pre-stressed levels. We found that this correlates with reduced mTOR activity and as a result decreases phosphorylation of 4E-BP1, which negatively regulates assembly of the eIF4F complex and cap-dependent translation. The decrease in mTOR activity and 4E-BP1 phosphorylation is associated with activation of AMP kinase, a negative regulator of mTOR, and in the case of some stress conditions, downregulation of signaling through key components of the PI3K pathway. Furthermore, we show that there is a subset of mRNAs that does not recover from UPR-induced translational repression, including those whose translation is particularly sensitive to loss of eIF4F, such as cyclin D1, Bcl-2 and MMP-9. Together these data implicate reduced mTOR activity and 4E-BP1 hypophosphorylation as a second, more restricted mechanism of translational control occurring somewhat later in the UPR.

Nuclear Matrix Protein 4 Is a Novel Regulator of Ribosome Biogenesis and Controls the Unfolded Protein Response via Repression of Gadd34 Expression

“…Consistent with prior reports (29), mTORC1 was repressed by ER stress in WT cells as illustrated by lowered phosphorylation of RPS6 during the time course of tunicamycin treatment (Fig. 3B).…”

Translational Control during Endoplasmic Reticulum Stress beyond Phosphorylation of the Translation Initiation Factor eIF2α

“…TSC2 phosphorylation at Ser 1387 inhibits Rheb-mediated activation of mTORC1 (73). The same authors did not observe this regulation in MEFs (35). In our studies, we also did not observe the activation of AMP-activated protein kinase in MEFs (data not Based on the data in this study, the eIF2␣-P signaling contributes to positive regulation of protein synthesis during chronic ER stress via the ATF4-mediated induction of amino acid transporters (26) and mTORC1-mediated translational control.…”

“…A recent report referred to the decline of mTORC1 activity during prolonged ER stress as a second node of translational control (35). The authors showed that the decreased mTORC1 activity was correlated with increased AMP-activated protein kinase activity and TSC2 phosphorylation on Ser 1387 , a known AMP-activated protein kinase phosphorylation site (76).…”

“…Recent reports showed that activation of AMP-activated protein kinase, a negative regulator of mTORC1, was correlated with decreased mTORC1 activity during prolonged ER stress (34,35). We hypothesized that the ATF4-induced amino acid transporter network contributes to protein synthesis rates during chronic ER stress by increasing mTORC1 activity.…”

Induction of fetal hemoglobin through enhanced translation efficiency of γ-globin mRNA

“…16,17 After 24 hours of Sal treatment, p-eIF2a recovered to control levels ( Figure 2B) and the polysome profile from Sal-treated cells was shifted less to the 80S fraction ( Figure 2D). During this recovery phase, the translation efficiency of g-and b-globin was increased as indicated by a significant shift to higher ribosome occupancy ( Figure 2E).…”